Simple Computer Imaging and Mapping - World Bank · 2016. 7. 17. · (List continues on the inside...

WORLD BANK TECHNICAL PAPER NUMBER 206 37592

Simple Computer Imaging and Mapping

Micha Pazner, Nancy Thies, and Roberto Chavez

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(List continues on the inside back cover)

WORLD BANK TECHNICAL PAPER NUMBER 206

Simple Computer Imaging and Mapping

Micha Pazner, Nancy Thies, and Roberto Chavez

The World BankWashington, D.C.

GIS World, Inc.Fort Collins, Colo.

Copyright i 1993The International Bank for Reconstructionand Development/THE WORLD BANK1818 H Street, N.W.Washington, D.C. 20433, U.S.A.

All rights reservedManufactured in the United States of AmericaFirst printing May 1993

Technical Papers are published to communicate the results of the Bank's work to the developmentcommunity with the least possible delay. The typescript of this paper therefore has not been prepared inaccordance with the procedures appropriate to formal printed texts, and the World Bank accepts noresponsibility for errors.

The findings, interpretations, and conclusions expressed in this paper are entirely those of the author(s)and should not be attributed in any manner to the World Bank, to its affiliated organizations, or tomembers of its Board of Executive Directors or the countries they represent. The World Bank does notguarantee the accuracy of the data included in this publication and accepts no responsibility whatsoeverfor any consequence of their use. Any maps that accompany the text have been prepared solely for theconvenience of readers; the designations and presentation of material in them do not imply the expressionof any opinion whatsoever on the part of the World Bank, its affiliates, or its Board or member countriesconcerning the legal status of any country, territory, city, or area or of the authorities thereof orconcerning the delimitation of its boundaries or its national affiliation.

The material in this publication is copyrighted. Requests for permission to reproduce portions of it shouldbe sent to the Office of the Publisher at the address shown in the copyright notice above. The World Bankencourages dissemination of its work and will normally give permission promptly and, when thereproduction is for noncommercial purposes, without asking a fee. Permission to copy portions forclassroom use is granted through the Copyright Clearance Center, 27 Congress Street, Salem,Massachusetts 01970, U.S.A.

The complete backlist of publications from the World Bank is shown in the annual Index of Publications,which contains an alphabetical title list (with full ordering information) and indexes of subjects, authors,and countries and regions. The latest edition is available free of charge from the Distribution Unit, Officeof the Publisher, Department F, The World Bank, 1818 H Street, N.W., Washington, D.C. 20433, U.S.A., orfrom Publications, The World Bank, 66, avenue d'Iena, 75116 Paris, France.

ISSN: 0253-7494

Micha Pazner is assistant professor at the University of Western Ontario. Nancy Thies is a researchassociate at the University of Western Ontario. Roberto Chavez is senior country officer in the CountryOperations Division of the Southern Africa Department of the World Bank.

Library of Congress Cataloging-in-Publication Data

Pazner, Micha.Simple computer imaging and mapping / Micha Pazner, Nancy Thies,

and Roberto Chavez.p. cm. - (World Bank technical paper, ISSN 0253-7494 ; no.

206)Includes bibliographical references.ISBN 0-8213-2467-51. Cartography-Data processing. I. Thies, Nancy. II. Chavez,

Roberto, 1947- . m. Title. IV. Series.GAl02.4E4P4 1993526'.0285-dc2O 93-13616

CIP

Abstract

Simple Computer Imaging and Mapping documents a set of techniques forcreating digital spatial databases, atlases, and wall maps. The techniques areuseful in operational, instructional, and research contexts involvingmanipulating and communicating spatial information.

The text has three main sections-"Input," "Processing," and "Output." The"Input" section describes how to acquire digital files by scanning maps and airphotos. The "Processing" section presents techniques for working with thedigital files: enhancing digital images to make the information in them moreaccessible, joining together individual scanned files to create images or mapmosaics, creating computer maps from scanned images, marking computerimages, and measuring objects shown in maps and images. The "Output"section describes techniques for presenting processed spatial data, both onpaper and on a computer screen. The topics include: creating large papermaps and images by joining together individual page-sized printouts,including maps and images in reports and other documents, and displayingmaps and images on the computer screen in a useful, quick, and attractiveway. The "Afterword" gives an example of how some of the techniquesdescribed in the book were actually used.

Appendixes include: a glossary, recommendations on how to maintain adigital database, some very basic information on digital satellite data,background information on the hardware and software used in developingthe techniques, and selected readings.

This publication describes imaging techniques. When applied to mapping,these techniques form a small subset of geographic information systems (GIS)technology, which deals with using computing equipment to acquire and usespatial data. The book describes a specific body of practical methods that haveproved to be relatively inexpensive, quick, and easy to learn, and that havedelivered usable products. The techniques make use of general-purpose,low-cost desktop computing hardware and software. Each techniquedescription includes a brief discussion of hardware and software and gives astep-by-step description of the process. Graphics are used extensively toillustrate processes and to give examples of results.

A basic limitation is in the accuracy and appearance of the maps and imagesproduced using these techniques. If cartographically finished, geometricallyrectified maps are needed, these techniques are not appropriate. There are,however, situations where having a map, even a rough map, is preferable tohaving no map at all. When availability and affordability are moreimportant than accuracy and appearance, these techniques can be useful.

ContentsForeword by Edward Echeverria viiiForeword by H. Dennison Parker xIntroduction and Summary xiii

4 Part 1: INPUT 1

1 Input by Scanning 32 Technique-Scanning Air Photos 13

3 Technique-Scanning Maps 21

3 Part 2: PROCESSING 27

4 Enhancing Digital Images 295 Digital Mosaics 396 Technique-Creating a Digital Mosaic Using Drawing Software 437 Technique-Creating a Digital Mosaic Using Image Processing Software 518 Creating a Computer Map from a Scanned Image 599 Technique-Creating a Computer Map in Raster Format 63

10 Technique-Creating a Computer Map in Vector Format 7111 Marking Maps and Images 7712 Measuring Maps and Images 81

Part 3: OUTPUT 89

13 Hard Copy Output-Printing Maps and Images 91

14 Technique-Creating a Printed Mosaic 9715 Including Maps and Images in Reports and Other Documents 10316 Screen Output-Viewing Digital Maps and Images 111

Afterword: An Example That Combines Techniques 115

Appendix A: Glossary 119

Appendix B: Taking Care of a Digital Database 123

Appendix C: Satellite Data 127

Appendix D: The Hardware and Software Used in Developing

the Techniques 129

Appendix E: Selected Readings 133

v

ForewordIn 1949 record-breaking rains washed thousands of adobe huts off the Guatemalanplateau, loosening their precarious balance on the steep-sided barrancas, pitching themheadlong into the canyons below. I was a young planner, responsible for rebuilding thehousing, schools, and services of the thousands left homeless. There were no maps, andaerial photos were either not available or were far too costly. This lack of spatial datamade our efforts to rebuild all the more difficult.

The planning and development tools available to us today to determine soil/terrainformations and guide us in building housing or plotting urbanizable limits represent aquantum jump over the earthy techniques available to me 40 years ago. In the interim, Ihave become adept at interpreting satellite photography to identify the geography ofdistant and recondite corners of the world. Though the space age provides maps thatwere otherwise unavailable, interpretation requires sophisticated instruments nottransportable to the field.

Between 1989 and 1991 I participated in the World Bank missions that led to thedevelopment of the techniques described in this publication. The purpose of themissions was to prepare an Urban Environmental Rehabilitation Project for Angola.The missions, which included as many as 10 consultants, were led by architect/plannerRoberto Chavez and included as team members Micha Pazner, a geographicinformation systems specialist and myself, a senior planner. The mission was acollaborative effort, carried out in the field in an exchange between the practitioners andbeneficiaries of the planning process. Government planners and local residents workedtogether with the World Bank team to find solutions to pressing environmental andsocial problems.

Though we lacked detailed surveys or coastal geodetic maps, the mission had toinvestigate resources for an expanded water supply for the Lobito-Benguela urbanregion. At that time, the population depended on limited ground water. A low weirhad already been built on the Catumbela River, 4 kilometers inland; it channeled asmall portion of the flow to irrigate the cane fields along the coast. Earlier studies byBulgarian consultants had proposed locating a new dam 120 kilometers upstream, acostly solution that exceeded the projected demand for water and the money available.The team turned to computerized mapping to search for a less costly solution. Asatellite image of the river was viewed on a portable computer; the relevant subscenewas selected and enhanced to show the steep cliffs, sand bars, and vegetation along theriver; and, finally, a printout was made-all in the field, and within a matter of hours.

Armed with a print of an enlarged portion of the satellite image, I led the team of localofficials and the Bulgarian consultants along the river. Several potential dam sites wereidentified on the image, prior to the field trip. We drove to the weir, some 20 kilometers

vii

away, in half an hour. The trek along the river passed through the narrow agriculturalverges with rich vegetation clinging to the escarpments on both sides, with crocodileslying in wait beneath the water's surface. One steep section on a footpath was notchedinto the side of the cliff. The experience was dramatic. In less than two hours, weverified a feasible dam site, some 3 kilometers upstream from the weir, one of thepotential locations we had marked on the satellite image. A medium height dam couldbe readily constructed at the point where the cliffs narrow, in a gap of less than 300meters.

Thus, with a minimum of manpower and a two-and-a-half-hour expedition, a greatpotential water resource was identified and blocked out in sketch form to establish apreliminary cost estimate, with a capacity adequate to supply the foreseeable needs ofthe region. The multiple benefits of controlling the periodic floods that assault thedownstream community of Catumbela and the possibility of hydroelectic power willalso be studied. Without the computer mapping, we would have spent considerabletime and money in trekking through an unknown terrain.

It is with great pleasure that I share with the reader the product of our joint endeavor,Simple Computer Imaging and Mapping. This new publication is a clear and concisehandbook that will enhance the skills of planners and expand their capabilities. Thesystem is relatively simple, and this publication makes the methodology as clear as anillustrated cookbook. It is readily accessible both to the experienced practitioner andthe novice in the field.

Using these new techniques, the scanning and processing of aerial photography,satellite images, or line maps has become incredibly fast and produces images ofbuildings, land forms, drainage, and vegetation of sufficient clarity and detail toprepare preliminary sketch plans in countries where there are no up-to-date geodeticsurvey maps. Maps produced by this system have been used to plan the expansion ofurban infrastructure and road development, as well as the analysis of regionaldevelopment and natural resources. The flexibility and diversity of applications ofdigital spatial data makes it useful for all urban planning problems. Scale is no longer abarrier; we can enlarge and reduce our maps at will. The ability to join scanned sectionsto create a mosaic is an important part of the capability. Digital enhancement of theimages is another technique available to improve interpretation.

Each step in the scanning and processing of the source material, whether photos or linemaps, is well illustrated showing results from line art or grayscale, permitting the userto select the procedure and produce the maps most suited to need. The creation ofcomputer maps in raster or vector format is demonstrated through techniques that areimportant to understand and fully exploit the variety of maps and data that can beproduced at relatively low cost.

This technical manual deserves the broadest circulation. It is accessible to people withlimited prior experience and offers techniques that are applicable to all forms of

viii

geography and a wide range of planning studies. It offers an essential tool for studentsand practitioners in the field.

Edward EcheverriaSenior Planner, World Bank (retired), and Planning Consultant

ix

I

ji

i

ForewordGIS World, Inc. is honored to cooperate with The World Bank in making available thisuseful introduction to computer mapping. Simple Computer Imaging and Mapping is anontechnical, practical guidebook to using inexpensive, digital remote sensingtechniques for land cover mapping. Its utility extends to anyone, anywhere on theplanet, who is concerned with mapping Earth for scientific, educational, or businesspurposes. The techniques described, as well as the data required to use them, areglobally available.

The title of the book belies the sophistication of the techniques explained. Seeminglysimple, the technologies of image scanning and processing in a microcomputerenvironment are advanced, both optically and electronically. They seem simple onlybecause of the sophisticated human-computer interface of the Macintosh computersused. In reality, the capabilities described took decades of advanced research anddevelopment to evolve. They are available only because of the rapid advances incomputer hardware that have been made in recent years.

More complex digital data analyses are not the point of the book, but they certainlywould be the next step for anyone who wishes to pursue them with more advancedsystems. This book is an outstanding introduction to further study, as well as a guide tomodern remote sensing methods that are useful on the ground immediately, with nofurther preparation. The authors are to be congratulated on a challenging job welldone.

H. Dennison ParkerFounder, GIS World, Inc.

xi

IIII

I

Introduction and SummarySimple Computer Imaging and Mapping documents a set of techniques useful formanipulating and communicating spatial information in studies or during projectpreparation or implementation. These techniques are also applicable in otheroperational, instructional, and research contexts.

Timely, accurate maps are important for many studies and planning and managementtasks. However, especially in developing countries, there are times when such mapsare inaccessible or even nonexistent. In such cases, methods for producing currentmaps locally, quickly, and cheaply can be valuable. This publication presents somecomputer-based methods for doing just that. It is a how-to manual describing a body oftechniques that can be used to produce inexpensive maps using generic, low-cost, user-friendly hardware and software.

Although the techniques are described in general terms and have general applicability,they grew out of the specific experience of World Bank missions during the preparationof the Lobito Benguela Urban Environmental Rehabilitation Project in Angola in 1989through 1991. It was found that there were virtually no maps less than 20 years old forthe Lobito-Benguela conurbation, which had tripled in size during that period. To fillthe need for up-to-date information, mission members sought to produce some mapsusing personal computers and peripheral equipment.

Satellite images were acquired in digital form, then enhanced and printed. Later, airphotos were scanned and used to create "image maps." With ground checks,population densities and housing and infrastructure standards were identified. Thismade it possible to determine, among other things, the number and location of about500 new communal standpipes and washbasins. Under the Bank-supported project, thelocal project team intends to apply these techniques to updating cadastral records,monitoring environmental change, and tracking the installation of improved pit-latrineslabs.

The hardware used included a Macintosh* computer, a scanner, a laser printer (allcapable of handling a minimum of 16 levels of graytones) and a mass storage deviceand storage media. The cost for the hardware started at about US$10,000; the softwarepackages used came to about US$1,500. In the body of the publication the hardwareand software is described in a general way (computer rather than Macintosh IIcx ordrawing software rather than MacDraw IMm); appendix B describes in more detail theactual hardware and software used in developing the techniques.

This manual describes imaging techniques. When applied to mapping, these techniquesform a small subset of geographic information systems (GIS) technology, which dealswith using computing equipment to acquire and use spatial data. No attempt is madehere to describe the many other, worthy techniques that make up GIS. It should be

xiii

clearly understood that the scope of this publication is narrow: it describes a specificbody of practical methods that have proved to be cheap, relatively quick, and easy tolearn, and that have delivered usable products. It is definitely not a review or summaryof all, or even many, of the current GIS techniques.

Another basic limitation is in the accuracy and appearance of the maps and imagesproduced using these techniques. For example, the techniques do not include methodsfor detecting and correcting inaccuracies and distortions that can be introduced duringdata acquisition and processing. If cartographically finished, geometrically rectifiedmaps are needed, these techniques are not appropriate. There are, however, situationswhere having a map, even a rough map, is preferable to having no map at all. Theexperience of the Lobito Benguela Urban Environmental Rehabilitation Project teamwas one such situation. When availability and affordability are more important thanaccuracy and appearance, these techniques can be useful.

The text has three main sections-"Input," "Processing," and "Output." In each section,techniques are discussed and, when possible, presented in a step-by-step fashion.Chapters 2, 3, 6, 7, 9, 10, and 14 give step-by-step directions and have a standard format.The first page gives a general description of the technique and the hardware andsoftware necessary to accomplish it. The second page shows, in pictorial form, the stepsmaking up the technique. The remainder of the chapter expands upon and illustratesthose steps. The first two pages of each of these chapters serve as a graphical quickreference or summary.

To implement the techniques in this book, the reader must have at least a basic masteryof the hardware and software required. The focus is on specific spatial processingtechniques rather than on computer literacy. There is a tremendous variety of productscurrently available and both hardware and software continue to change rapidly.Therefore, the techniques are presented in a general way, rather than tying them to thespecific sequence of steps required by a specific configuration of hardware andsoftware.

Although maps, air photos, and satellite images are the focus of this book, many of thetechniques are useful for pictures in general. Black-and-white photographs can betaken with a camera, scanned, and then enhanced, included in documents, printed as amultipage poster, etc. It is also possible to scan regular color photos; modern (24-bit)image processing software provides special features to convert color photo images intograytone images. This is precisely how the photographs that appear in the manualwere generated and processed-from both black-and-white photos and color photosand slides.

The "Input" section describes how to acquire digital files by scanning maps and airphotos. The "Processing" section presents techniques for working with the digital files:

xiv

enhancing digital images to make the information in them more accessible, joiningtogether individual scanned files to create images or map mosaics, creating computermaps from scanned images, marking computer images, and measuring objects shown inmaps and images. The "Output" section describes techniques for presenting processedspatial data, both on paper and on a computer screen. The topics include: creating largepaper maps and images by joining together individual page-sized printouts, includingmaps and images in reports and other documents, and displaying maps and images onthe computer screen in a useful, quick, and attractive way. The "Afterword" gives anexample of how some of the techniques described in the book were actually used.

The appendixes include: a glossary, recommendations on how to maintain a digitaldatabase, some very basic information on digital satellite data, background informationon the hardware and software used in developing the techniques, and selectedreadings.

AcknowledgementsPreparation of this book was made possible by the support of The World Bank, whichcommissioned the work for the Lobito Benguela Urban Environmental RehabilitationProject (Angola). Much of the work that is documented in this book was done with theample and much-appreciated cooperation of the Comissariado Provincial de Benguela,Gabinete Provincial do Plano (Angola). The Department of Geography, Faculty ofSocial Science of the University of Western Ontario provided the facilities in which thework was done. Apple Canada donated some of the equipment. Robert J. Gammon,K. Chris Kirby, and Dr. Cheryl M. Pearce, all of the University of Western Ontario,provided valuable assistance.

xv

Part 1: INPUT 4

Often, the first step in using a computer is to translate information from human-accessible form into computer-accessible form. When using computers for mappingapplications, this generally means using existing maps, air photos, and satellite imagesto create digital (i.e., computer readable) files. This is referred to as input, informationin a form that the computer can use.

There are a number of ways to translate the information in a map or image intocomputer form. These include (but are not limited to) tablet digitizing, on-screendrawing, typing in spatial information as lists of numbers, and scanning. Each one ofthese methods has advantages and can be best for specific types of applications.

Here, discussion is limited to one type of input, scanning. There are several reasons forthis limited approach. Rather than provide an overview of mapping techniques and thehardware and software that can be used, we describe a body of specific techniques. Thetechniques were chosen on very practical grounds: they have proved to be relativelyinexpensive, they can be implemented with widely available hardware and software(which can also be used for nonmapping applications), and and they are easy to learn.In other words, scanning meets the requirements of simplicity and low cost and hence isthe starting point for the rest of the computer imaging techniques that make up thispaper.

-|!- -~ - ----

91MjX . -w.

.1eEquimen use fo-cnnn.Te scanri otelfto hoptr

Chapter 1Input by Scanning

z- Scanning is a method of recording graphic information in digital

form. It can be used to capture the spatial information in airphotos and paper maps as well as other graphics, including

* J- sphotographs, drawings, and charts. Scanning is the startingpoint for most of the techniques described in this book. This

- * chapter gives an overview of the process and chapters 2 and 3present, in more detail, the steps necessary to scan air photosand maps.

What are the pros and cons of scanning? The advantagesinclude:

* Scanning requires little training.

* The scanning process is relatively quick.

* Scanners and scanning software are widely available and are fairly inexpensive.

* Scanners can be used to produce digital forms of images as well as maps and canalso be used for other graphics, such as drawings, photographs, and diagrams.

* Scanning can capture in picture form most of the information present in theoriginal.

* Scanning does not harm or alter the original.

Naturally, scanning also has limitations. The type of scanning discussed in thispublication produces output in pixel, or raster, format. In this format the information isdisplayed as an array of square cells each representing a square parcel of land, or insome cases, a sampling point within a square parcel of land. If some other format isneeded, further processing is necessary. In particular, post-scanning processing isalmost always required when scanning line maps. Depending on the size of thescanned image and the type of processing, this can be difficult and time-consuming. Ina sense, scanning is a compromise technique. It is very good for images such as airphotos. It is less than ideal for line maps.

How is scanning done? The commonly available type of scanner that is best forscanning maps and images is a flatbed scanner, a piece of equipment that resembles aphotocopier. It has a glass scanning window on which a graphic is placed face down.

3

4/ Chapter 1: Input by Scanning

When the graphic is scanned, a bright light passes over it. The scanner sees the graphicas an array of pixels, each having a color or shade of gray. The different colors orshades of gray reflect different amounts of light. The scanner assigns a number to thereflectance of each point. The array of numbers resulting from scanning becomes thedigital image file, which can then be processed, displayed as an image on the screen, orprinted.

What is the result of scanning? Scanning results in a digital image file, a computerpicture of whatever is scanned. The picture is in pixel or raster form, as if the originalmap or image has been divided into a grid of square cells. When displayed on thescreen, each cell has a color or shade of gray and taken all together, the cells form apicture. If the cells are very small, the viewer is not even aware of them. If the cells arelarger, the viewer may see some roughness in the picture or even see the cellsthemselves. The fineness or coarseness of the grid is chosen during the scanningprocess.

Air photo scanned in -agrayscale mode. e -Note the ship.

"I

The result of scanning an air photo is anarray of pixels or square cells. When thescale of the scanned image is large enough,the individual cells can be seen.

Scanned image of /the ship enlargedto show the cells.

From the computer's point of view, the digital image file is an array of numbers. Whilethe original map or image existed only as a graphic, the digital file has two forms: thevisual, graphic form seen on the screen or as a printout; and a numerical form that thecomputer can manipulate. The numerical representation of the information makesvarious sorts of processing possible. Images can be enhanced, measured, edited,combined, resized, and printed.

When should scanning be used? Scanning has to start with an existing map or image.There are times when it can be a very useful technique; there are also times whensimply photocopying the original might be better. Scanning is advisable under thefollowing conditions:

Simple Computer Imaging and Mapping /5

* When the original maps or photographs are available on a very limited basis.

Rather than guessing at the number of photocopies that may be needed in the

future, a single digital "copy" can be used to produce printouts as they are

required.

* When the ability to enhance or otherwise process the resulting image is

important. For example, the digital files resulting from scanning air photos can

be enhanced to extract more information or joined into a seamless digital mosaic.

* When the ability to get printouts at different scales or of selected portions of a

map or image is important. In general, there are more options available when

printing a digital image file than when copying a paper map or image.

* When the resulting digital file will be used repeatedly. That use may be repeated

on-screen viewing, frequent printing, or inclusion in a spatial database. If the

map or photo is to be used infrequently or for a limited purpose, it may be better

to just photocopy the original.

How is scanning done? Scanning is done by (1) choosing a number of scanning

parameters, (2) physically scanning the map or image, and then (3) saving the resulting

digital file. In this context, parameters are the characteristics of the scanning process that

determine how it will procede. The parameters chosen include scanning mode,

scanning resolution, threshold, brightness, contrast, scale, and scanning area. Once the

map or image has been scanned, the digital image file has to be saved. Most scanning

software allows the image files to be saved in more than one format; the format or

formats chosen in each case depends on how the images are going to be used.

r-J Mode

Choose Resolution

How is scanning done?


How is scanning mode chosen? The scanning mode determines how the scannerinterprets the material that is scanned. Most scanners can scan in both line art andgrayscale mode. Other scanners have other modes and can, for example, scan in color.This publication discusses only the line art and grayscale modes.

I.. .v;' - v--l - In line art mode, everything is seen as either black orci white, with nothing in between. In grayscale mode,the scanner sees a range of tones, going from pure

10 w- -- white, through a series of darker and darker grays, toOURAIR-Amo x5- ,1 pure black. The number of grays that are detected

depends on the scanner. Scanners that can detect 16

different intensities of gray are said to be capable of 165 L....J levels of gray. Scanning in line art mode is usually

quicker and the resulting files are smaller than with-II II , grayscale (or color) scanning.

City planning map scannedin line art mode The choice of scanning mode depends on how

information is represented in the original. Air photos,where the information is represented as patterns ofwhite, gray and black, need to be scanned in grayscalemode. In general, it is necessary to have a scannercapable of at least 16 levels of gray to get adequategrayscale results.

Maps can be scanned in either line art or grayscalemode. The choice of mode depends on the specificmap. Maps that are drawn in dark ink on light paperare best scanned in line art mode. There is a goodmatch between how the information is presented inthe paper map and how the scanner "sees" it. Forother tvpes of maps, such as thematic maps that

Contour map scanned combine lines with shaded areas or road maps thatin grayscale mode use primarily lines and points but in different colors,

the choice is less clear. Sometimes line art scanningwill capture enough information to produce usefulresults, sometimes it will not. The choice reallydepends on the specific maps and on how theresulting digital files will be used.

1n 2+ ^In most cases, the information recorded in scannedfiles needs further processing. For example, the files

* may be imported into a drawing program to serve asAir photo scanned in grayscale mode the basis for creating computer maps. The drawing


program may handle a line art image very efficiently but not be able to accept agrayscale image. When there is a clear understanding of what programs will be usedand what their capabilities are, that should be a factor in choosing scanning mode.

What is scanning resolution? In a very general way,resolution is a measure of the amount of detail thescanned image will show. The scanner looks at a Lower resolution I I I I IIgraphic as if it were divided into a grid of tiny squares.It measures the light reflected from each little square.Low resolution means that a small number of relativelylarge squares is used while high resolution means thata larger number of relatively small squares is used. Higher resolutionScanning resolution is often measured in dots per inch(dpi). If a graphic is scanned at 100 dpi, then the ...............scanner "sees" 100 dots or points in each horizontal andvertical inch of the graphic.

The same photo, scanned attwo different resolutions. In -the photo on the right, I X -buildings can be identified *easily. It is much more Ldifficult to make thatidentification from the a ,Zs;image on the left.

Note: The 150 dpi scan L - Lresults in a smaller image. -

The image was later *enlarged and brought to the |same size as the 300 dpiscan for comparisonpurposes. Scanned at 150 dpi Scanned at 300 dpi

The advantage of scanning at a higher resolution is clear: more information is recorded.The disadvantage follows logically; more information requires more memory andstorage. And, because of the greater demands on memory and storage, doing anythingwith the images-viewing them, processing them, storing them, printing them-takesmore time. Even scanning them takes longer.


The following graphics give an idea of what is involved in this trade-off. Four imagesof the same building are shown, the results of four different scans of the same photo.Each dot or square that the scanner detected is shown at the same size. Even thougheach image shows the same thing, the number of dots used to show it is different, andas a result each image is not only a different size but also shows a different amount ofdetail. The lower the resolution, the fewer the dots, the coarser and the smaller theimage. The size of the image file is directly related to the number of dots shown in theimage. It's easy to see that more information requires more storage space.

... - -B *YZ

4 ;. a L

J-1~

150 dpi 200 dpi 250 dpi 300 dpi

No one resolution is always best. Often experimenting with a typical photograph ormap is the best way to choose resolution. The decision depends on four things:

1. Goals. What are the scanned images going to be used for? For instance, whenscanning air photos, is it enough to see a general pattern of neighborhoods, or is itnecessary to see individual buildings?

2. Data. What is the quality of the graphics being scanned? With air photos, whatfeatures can be distinguished and what features are indistinct? How detailed are themaps? At what resolution can you be sure of capturing the finest lines used in themaps? When does scanning at a higher resolution give more noise (pixels that arethe result of the scanning process rather than part of the original information) butvery little additional information?

3. Tools. What can the equipment do? At what resolutions can the scanner scan?

4. Resources. How much computer memory and storage are available and canreasonably be dedicated to the scanned images?


What other scanning parameters are there? There are a number of other scanningparameters that are chosen when a graphic is scanned. These include scale, scanningarea, threshold (or its equivalent), and contrast and brightness.

The scale or size of the scanned image can be selected prior to scanning. This option isusually labelled "scale" or "reduce/enlarge." It is a way to determine the size of thescanned image. This can be useful when it is necessary to fit a scanned image into apredefined space.

Scanning area is something else that can often be chosen - '.. . I:: ;;=lat the time of scanning. The largest area that can be -.

scanned is the area of the glass scanning window, 2 ,which may actually be smaller than the map or imageto be scanned. Sometimes it is useful to scan a smallerarea. When the graphic being scanned is smaller thanthe scanning window, there is no point in including 6 . -anything but the graphic in the scan. And, there can be .times when only a portion of a map or image is needed 8.and it is useful to scan just that portion. -.

Threshold is used when scanning in line art mode. This .. - .>setting tells the scanner when to classify a point it 12

scans as black and when to classify it as white. It isparticularly important when the background of the -. . .. .map is not white. For example, when the maps being Selecting a scanning area smallerscanned are printed on blueprint paper, everything on than the scanning windowthem is a shade of blue. The threshold setting will tell while in preview mode

the scanner when a shade of blue is dark enough to becalled black and when it is light enough to be called white. Because there can be greatvariation between maps, the threshold setting should be determined for each mapindividually.

The same map, scanned at different , ';threshold settings \


Contrast and brightness settings are useful when scanning a map or image in grayscalemode. The contrast setting determines whether the scanner "sees" the graphic as apattern of primarily black and white with a few shades of gray in between or as apattern of varying grays with less pure black and pure white. The brightness settingcontrols the overall darkness or lightness of the scanned image.

The combination of contrast and brightness makes a difference in how clearly differentfeatures in the image can be seen. By experimenting with different settings it is possibleto find which ones produce the best image for a given purpose. A good rule of thumb isto start with the default settings (the ones that the scanner assigns automatically.) Theseare often close to optimal.

- r

The same map,. = scanned in

grayscale mode,Low Brightness High Brightness using different

settings forbrightness andcontrast

Low Contrast High Contrast

How are the scanned files saved? To save a scanned image file, three things have to bespecified: the device on which it is saved, the format of the file, and the name of the file.

Because image files are generally too big to be saved on floppy disks, they should besaved on a mass storage device that is specified as the device. Some form of off-linemass storage is also necessary for backup and archiving. Removable mass storagedevices can serve both purposes.


The format in which the files are saved is important because different software packagescan read or import different formats. The formats chosen determine, to some extent,what can be done with the image files because format determines which softwarepackages can be used. For instance, PICT and TIFF formats are two of the manyformats available. PICT format is often best for drawing programs, while TEFF format ispreferable for image processing programs. Another consideration when choosing thefile format is that some, but not all, formats can later be used to create an image file inanother format. For example, if a scanned image is saved as a TIFF file, that TEFF filecan be used to create a PICT file. However, the reverse is not true. If a scanned image issaved as a PICT file, the PICT file cannot be used to create a TIFF file without losingsome of the information. Sometimes the most practical choice is to save a scannedimage in more than one format.

File names should be chosen so that they can be readily understood and related to theoriginal maps or photos. Although choosing file names may seem a very commonplacematter, a simple, logical naming system can simplify repeated use of the resultingimage files. For instance, if a group of air photos over a city is scanned, the scannedfiles may be named CITYNAME-### where "CITYNAME" stands for the name of theparticular city and "###" stands for the photo number. A naming system like this hasthe advantages of being easily understood by people other than the individual doingthe scanning, as well as being expandable should other photos be scanned. Recently,more and more computer systems support longer and more user-friendly file names.

What other considerations are there when scanning? This discussion of scanning hasbeen very general and has touched only upon those aspects that are common to mostscanning jobs. The two chapters that follow give more specific procedures for scanningair photos and maps.

Chapter 2Technique-Scanning Air Photos

-,, - Air photos can provide an important source of spatial

* - w, information, especially when there is significant, rapidchange taking place. Air photos show much more detailthan satellite data. They are also less expensive and ofteneasier to obtain than data from ground surveys, although

- ground-based verification of air photos is always advisable.-Scanning air photos to produce a digital version can expand

their usefulness. The digital versions can be enhanced usingI Ximage processing techniques, can be viewed and printed in

Aa wide variety of formats, and can be used as the basis for. ncomputer maps.

The Hardware

A personal computer that has adequate random access____l_memory (RAM) and a monitor that is capable of displaying.. .at least 16 levels of true gray.

Personal computer

A flatbed scanner that is capable of scanning at least 16levels of gray.

Scanner

\ lA removable mass storage device that is capable ofrecording the large files resulting from scanning. TheRemovable device should be usable for both on-line (i.e., working)

storage and for backup.

The Software

I T ) System software, which is the basic software that makes thecomputer run and allows other software packages to beused. It is contained on the computer's start-up disk.

System software

|] L )Scanning software which allows the scanner to "talk" to thecomputer. Every scanner requires some form of software todo so.

Scanning software

13

14/Chapter 2: Technique-Scanning Air Photos

Scanning Air Photos

X2 5-3 5-5 -58

Organize photographs 547 5-i 54.

Create delineation index I- -h--

Lower resolution

(2 Decidescanning resolution * r

X fl Higher resolution

3 e Scan photos

Choose scanning parameters for each photo

I

BrightnessScanning area and contrast


The flowchart on page 14 shows the basic procedure for scanning air photos. First, thephotos need to be organized and an index created. Second, one photo should beselected and scanned at different resolutions to determine the best scanning resolution.Third, the photographs are scanned. Other scanning parameters are chosen on a case-by-case basis as each photo is scanned, and the resulting digital files are saved.

---- Air photos are usually-5-32 5-34 5-53 5-58 taken and used in groups

5O47 5-60 5-48 that cover an area of1 )Organize photographs interest such as a city.Create delineation index 5-55 5-36 5-33 terest phots are-- Typically, the photos are5-s41 s4 s49 numbered, although the

numbering sequencedoes not necessarily have

a direct relation to the location shown in the photo. When a group of photos is scanned,they should first be laid out on a flat surface and put together like a jigsaw puzzle toshow a complete picture of the area of interest. It is helpful to use weights to hold thephotos flat and in place during this process.

When the photographs are organized,a delineation index can be created. A - (2-718)delineation index shows how the -TTI4}IT7Jphotos are laid out, with the number of - -each photo noted in position. The - - -j--- -5-5-5-delineation index is used to locate aphotograph in relation to the other ----- E- - -photographs or within the overall area. Lobito, Angola Lobito, AngolaOnce the index has been completed, Air Photos Air Photosthe photographs themselves can be 31 August 1984 31 August 1984removed. Blank delineation index Delineation index with

photo numbers notedWhen a large number of photos overseveral areas or cities are being scanned, it is useful to create a form for the delineationindex. The form is just a grid of empty squares that can be photocopied and then filledin for each city.

;- 2 The second step is toe rdecide scanning

Lower resolution |resolution. As2 ) Decide discussed in chapter 1,

scanning resolution H I this is generally doneHigher resolution experimentally and by


considering four things: the intended use of the scanned images; the quality of theoriginal photographs; the scanning resolutions possible with the scanner being used;and the computer time, memory, and storage that can reasonably be set aside for theseimages.

.i. b~- ~ - jL--

The same air photo scanned at three different resolutions.

A single photograph with a typical range of graytones and features should be selectedand scanned at different resolutions. The resulting image files show how much detailcan be seen at each resolution and how large a representative file will be.

The photos can now be scanned. Photographsare made up of patterns of white, different

3 ) Scan photos 0L . shades of gray, and black and should beIu scanned in grayscale mode. Although there are

l_ always exceptions, air photos are generallyscanned at a scale of 100 percent, without being

either reduced or enlarged. Because air photos covering a given area are typically usedas a group, the same scale is normally used for all of them. Other scanning parameterssuch as scanning area and brightness and contrast are decided at the time of scanning.

The scanning area that is chosen depends on the intended use of the scanned photos.Two of the most common uses for air photos are for making mosaics-large imagescreated by joining together the scanned photos of a given area-and for viewing asstereo pairs. Air photos are often taken so that there is overlap between adjoiningphotos. Typically there is 60 percent overlap on the sides of each photo and 40 percentoverlap on the top and bottom. This makes it possible to align adjoining photos whenforming mosaics; it also makes it possible to use the photos for stereo viewing. In stereoviewing, two photos of the same area, taken from slightly different positions, areviewed simultaneously. The human eye and brain integrate the two images, resultingin a three-dimensional effect. All unprocessed air photos have some distortion due to,among other things, the camera lens. Photos are most accurate at the center andincreasingly distorted toward the edges; the greatest distortion occurs in the area ofoverlap. The issue of distortion in air photos is also touched upon briefly in chapter 5.


If the scanned photos are to be joined together in amosaic, the objective in selecting the scanning area is tominimize distortion while leaving enough overlap tomatch up adjoining photos. In other words, the i -

scanning area should be the center of each photo (i.e., -cthe least-distorted part of the photo) with only a -,

minimum amount of overlap. If the scanned photos areto be used to produce printed images for stereoviewing, the entire photo should be scanned.

Often air photos in a set vary noticeably in theirbrightness and contrast. This variation can be caused bya number of different things. The photos are taken from Selecting the scanning areaa moving airplane, hence sun angle and cloud coverchange from photo to photo. Differences in processing can also produce differences inthe appearance of the photos. When the photos vary significantly, scanning brightnessand contrast may have to be chosen individually for each photo.

Selection of brightness and contrast is basically a trial-and-error process. The intendeduse of the images can affect which settings are chosen. For example, if populated areasare of the most interest, choose the settings that show the greatest detail in these areas,even at the possible expense of other areas, such as forested terrain. Quite often, it willnot be necessary to change the default settings very much to get an improved image.

Lower brightness Higher brightnesst%'* q.1 - I w

Higher brightnessand contrast

Lower contrast Higher contrast


Sometimes special cases come up at the time of scanning. These include photos that arelarger than the scanning bed of the scanner and photos that are not aligned with eachother. When the photos are too big to be scanned in one pass, the scanning should startat a corner. Once the corner area has been scanned, the photo should be moved so thatthe area now on the scanner includes a narrow strip of the previous area for overlap.Repeat this process until the entire photo is scanned. Take care whenever the photo ismoved to align it very precisely so that all the scanned portions of the photo are alignedwith each other.

In some cases, the air photos in a given group are not aligned with each other. Whenthis happens, one photo is chosen as the standard to which all the others are aligned.The "standard" photo is scanned first.

Photo chosen as Then, one of the neighboring"standard." All photos is selected. The secondothers aligned to photo is manually aligned to thethis one "standard" photo and taped in

place using tape that can beremoved without harming thesurface of the photos. Then thephotos are turned over and thelines formed by the edges of the"standard" photo drawn on theback of the neighboring photo witha pencil and ruler. These lines arethen used to align the neighboringphoto when it is scanned.

Lines drawn on theback of the nonalignedphot Area to be scanned

(includes overlapwith previous photo)

l Il ~Standard"t\ X~~photol! -

When the nonaligned photo is scanned, the scanning area is selected to overlap with theprevious photo and to have a standard, rectangular shape. The process is then repeatedwith the other photos, always continuing the lines formed by the edges of the"standard" photo. The farther a photograph is from this "standard," the easier it is forerrors to appear. The alignment process requires patience and care to keep distortionsfrom showing up.


After a photograph has been scanned, save the resulting image. When saving an imagefile, three things have to be specified: the name of the file, the device on which it is saved,and theformat of the file.

Choose file names that can be readily understood and related to the numbering systemused for the photographs. Because the image files are usually quite large, they aregenerally saved on a mass storage device that is specified as the device. The format inwhich the files are saved is important because different software packages can read orimport different formats. The formats chosen for the image files determine, to someextent, what can be done with them because they determine which software packagescan be used.

When scanning is complete, the air photographs are recorded in digital form and savedon mass storage devices. A major portion of the information in the air photos iscaptured with the photos themselves in their original condition. The capturedinformation is available for printing and further processing. However, digital filescannot record all of the data present in the air photos; any translation process involvessome loss of information. The choice of scanning resolution and the number ofgrayscale levels available are particularly important in this respect. On the other hand,digital images can be enhanced, making the information more available and easier touse. There is also great flexibility in printing the digital files. Hence, the digital files canactually prove to be, in some ways, better tools than the original photos.

ii

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Chapter 3Technique-Scanning Maps

Scanning is one method of capturing the information in anexisting paper map. The process is very similar to that usedfor air photos (see chapter 2). The result is a digital file thatcan be viewed, processed or printed or used to create acomputer map. Scanning is useful when the paper maps areavailable only on a limited basis or when the digital form isdesirable because of the flexibility it allows.

.S MURRAPAWA4J

The Hardware

A personal computer that has adequate RAM.

Personal computer

A flatbed scanner that is capable of scanning line artgraphics.

Scanner

A removable mass storage device that is capable of

Removable recording the large files resulting from scanning. Themass storage device device should be usable for both on-line (i.e., working)

storage and for backup.

The Software

L System software, which is the basic software that makes the_ computer run and allows other software packages to be

used. It is contained on the computer's start-up disk.System software

* []Scanning software that allows the scanner to "talk" to thecomputer. Every scanner requires some form of software todo so.

Scanning software

21

22/ Chapter 3: Technique-Scanrnig Maps

Scanning Maps

Line art modeDecide

\<Jscanning mode -g ,5

scai od - Grayscale mode

2 Decide |h I.A-ha*| Lower resolutionscanning resolution Et M4UAtrLA Higher resolution

(3 )Scan maps X

Choose scanning parameters for each map

F, ~' - ILine art V\7 Grayscale e - -

Choose threshold Choose contrastand brightness


Scanning maps is done in three steps. First, the scanning mode is decided, based on themaps themselves and on their intended use. Next, scanning resolution is decided,generally by experimentation. Third, the maps are scanned and saved. As each map isscanned, other scanning choices, such as scanning area, brightness and contrast, scale,and threshold, are decided.

Scanning mode, discussedin chapter 1, determines

Line art mode how the scanner( 1 A Decide interprets what it sees. If\ 1) Jscanning mode: a map is scanned as line

[ | Grayscale mode art, everything in thatmap is seen as either blackor white, with nothing inbetween. If the same map

is scanned as grayscale, the scanner sees it as a series of tones ranging from pure white,through progressively darker and darker grays, to pure black.

Map scanned in line art mode Map scanned in grayscale mode

The scanning mode should match the way spatial information is shown in the papermaps. Are the maps primarily lines and dots drawn in dark ink on light paper? Thenline art mode would be the logical choice. Are colors and shaded areas essentialfeatures of the maps? Then grayscale mode would probably be more effective.Sometimes the choice is obvious and sometimes there are benefits and disadvantages toboth modes. When making a choice, there are several things to consider:

* Scanning in line art mode is quicker and produces smaller files.

* Sometimes, because of size or shape, it is desirable to scan a map rotated 900from the normal orientation. If it is necessary to rotate the resulting digital imageback to the original orientation, it is much, much easier to do so with a line artimage than a grayscale image.

* Line art images can be successfully imported to a greater number of softwarepackages than can grayscale images.

24/ Chapter 3: Technique-Scanning Maps

None of these advantages, though, should be decisive if scanning in line art mode doesnot successfully capture the information in the original map.

Scanners offer a range

/ L A Lower resolution of scanning resolutions.2 Decide Lo r l n Experimentation is

\ ,. scanning resolution EMUATALA Higher resolution generally the best_ _ _ __ -- way to choose the

optimal scanningresolution for a map or maps. When scanning in line art mode, the scanning resolutionhas to be chosen so that the scanner will "see" the finest lines on the map. To choose thebest scanning resolution, select one map, with a typical range of lines, and scan it atdifferent resolutions. Compare both the quality of the scanned images and the sizes ofthe scanned files for different resolutions.

v1 v

Map scanned at 75 dpi Map scanned at 300 dpi

Notice that at the lower scanning resolution, parts of the text arenot picked up and the lines appear thinner.

When scanning in grayscale mode, the goal is a scanned image showing the smallestfeatures of the map clearly. Again, choose a representative map and scan it at differentresolutions to see both the quality and the size of the resulting images.

In general, the choice of resolution depends on the scanner's capabilities, the qualityand clarity of the original maps, and the amount of computer memory and storagespace available for storing the scanned files. The end-use of the scanned informationshould also be considered, as file size affects every type of manipulation-printing,displaying on the screen, processing, and so forth.


When doing experimental scans, adjusting other scanning parameters, such asthreshold, may be necessary to get a clear image to evaluate. The flowchart on page 22describing the scanning process separates each type of decision into a self-containedstep: decide mode, decide resolution, decide other parameters. In reality, the procedureis not always so orderly and sequential. It might be useful to experiment with thresholdor with brightness and contrast while choosing resolution and then fine-tune thoseparameters while scanning each individual map. When an individual scanning taskrequires a change from the general pattern described here, common sense should rule.

After scanning mode and resolution aredecided upon, scan and save the maps. At this

3 ) Scan maps point, scanning parameters are set and other-6 situations-such as maps larger than the

l_ scanning window-are dealt with. Scanningparameters were discussed in chapter 1 but aresummarized here.

Threshold-or an equivalent function-is used in line art mode and tells the scannerwhen to classify a point it scans as black and when to classify it as white. Because therecan be great variation between maps, the threshold (or equivalent) setting should bedetermined for each map individually.

-. I... .B J1Lfl4UANHA

Gradually decreasing threshold settings for scanning a map printed on blueprint paper

Contrast and brightness are used in grayscale mode. Contrast determines whether blackand white predominate in the resulting image, or the image is composed mainly ofshades of gray. Brightness controls the overall darkness or lightness of the scannedimage.

Scale allows the user to control the size of the scanned image. In general, the defaultchoice is 100 percent-the image is neither reduced nor enlarged.

Scanning area can be as big as the scanning window or just a portion of that area.Sometimes choosing to scan just the relevant portion of a map can reduce overhead-

26/ Chapter 3: Technique-Scanning Maps

the time, and the computer memory and storage space needed to create, store, and workwith the resulting digital file.

Quite often, the maps beingscanned are larger than thescanner bed and must bescanned in sections. In such Scan in four A B Overlap betweencases, it is very useful to pieces, scanned piecesestablish a standard scanning starting atorder and labelling scheme for the upper - N

the sections. For instance, one cornerfChsuch scheme is to scan the mapsstarting at the upper left-handcorner and go across to the right,then start again directly below the first scanned portion. The sections might be labelledby adding letters of the alphabet to each file name, in order: CityMap-A, CityMap-B,CityMap-C, and so on.

Each time a large map is realigned to scan another section, the edge of the map must bealigned very precisely with the edge of the scanner so that all the sections of thescanned map will be properly aligned with each other. Take care to allow a smallamount of overlap between the sections being scanned so that the sections can berejoined into a complete digital image.

Once scanning is complete, save the digital file. When saving the files, thefile name, thedevice on which it is to be saved, and the fileformat all have to be specified. File namesshould make it easy to relate the scanned file to the original map (without having to askthe person who did the scanning.) If there is no clear-cut way to do this, an index of filenames and their contents can avoid a lot of frustration on the part of those who have touse the files.

Most scanned files are too large to be stored on floppy disks. They are usually saved onremovable mass storage devices, which are specified as the device when saving.

Scanning programs generally offer more than oneformat for saving files. The formatchosen depends on how the scanned files will be used. For instance, a drawing orpresentation program might require files in PICT format, while an image processingprogram should have files in TIFF format.

When scanning is complete, the results are image files that record the maps in digitalform and are saved on a mass storage device. The image files can be viewed, printed,and used as the basis for computer maps. The original maps will not be changed in anyway.

Part 2: PROCESSINGOnce input is complete and a digital file (or files) exists, the file must be processed. Inits most general sense, processing means either changing a file or using it to create a newfile. The goal is to create a file that is more useful than the original. Processing canmean (among other things) modifying the visual appearance of a file, combining two ormore files, or extracting information from or adding information to an existing file. Thetechniques discussed here are digital image enhancement, creating digital mosaics,using scanned images to make computer maps, and marking and measuring computermaps and images.

It is in processing that the advantages of having spatial data in digital form becomeapparent. After all, scanning a map or air photo is rather like going through acomplicated photocopying process. If the result isn't potentially more useful than theoriginal, it is hardly worth bothering with. One of the advantages is havingmanipulation abilities that are not available with paper maps and images. Digitalenhancement, for example, can be used to restore to a useful form a digital version of afaded photo. Other advantages include increased flexibility and decreased cost. Forexample, individual scanned air photos can be combined digitally into a seamless wholeand used as a source of low-cost, custom-sized and -scaled paper image maps.

The techniques presented here are by no means exhaustive-there are many otherprocessing techniques that can be applied to digital spatial data. In fact, there is anentire area of study, known as geographic information systems (GIS) technology, thatdeals with acquiring, processing, and analyzing digital spatial data. The techniquespresented in Simple Computer Imaging and Mapping are a very small subset of GIS andemphasize imaging techniques, rather than, for example, line-based or vector graphics,or data in text or numeric form. These particular imaging techniques were chosenbecause they are basic and useful, they can be done with commonly available software,and, in the basic form presented here, they can give results with a minimum ofspecialized training.

A digital image being viewed 1-and manipulated.

27

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Chapter 4Enhancing Digital Images

g ' j What is image enhancement? Image enhancement is a termF ;that refers to techniques applied to digital images to change

their appearance and make them easier to interpret. Usually,the interpretation is done visually-someone looks at animage, sees patterns of white, black, and grays, and is able toidentify features in those patterns. (Images can also be in

t color, but this publication deals only with black-and-whiteimages.) Because the success of an enhancement depends onhuman interpretation, choosing and judging an enhancementtechnique is subjective. The test of an enhancementtechnique is simply: "Is the enhanced image a better pictureof what is actually there?"

l HHow is enhancement done? A digital image, whether ascanned air photo or a satellite image, exists in two forms.Inside the computer, these images are arrays of numbers. On

the screen, the images are arrays of display pixels. Each pixel is a small square that iseither white, black, or a shade of gray and represents an area on the ground. Thedifferent graytones are generally referred to as levels of brightness. Each level ofbrightness is associated with a numerical value. When an image is enhanced, the valuesrepresenting the different levels of brightness are manipulated according to differentmathematical formulas. The result is an image with a different distribution ofgraytones that ideally shows the features in the image more clearly.

Enhancement is generally done by specialized software called image processing software,although other types of software, such as geographic information systems, can includeimage processing capabilites. The process is accomplished using mathematics, but theuser does not need to understand-or even be aware of-the mathematics. Typically,the user chooses an enhancement technique from a menu or uses a graphical displaysuch as a histogram to apply it to the image. The software displays the enhancedimage, and the user can decide how successful the enhancement is.

What is a histogram? Histograms are specialized bar charts that show frequency ofoccurrence. Histograms are used extensively in image enhancement to help decidewhat enhancement to apply and to show graphically the difference between the originalimage and the enhanced image. Histograms are also used by many image processingpackages to actually apply enhancements. In any image, a count can be made of howmany pixels are black, how many are the darkest shade of gray, how many are the nextdarkest shade of gray, and so on. The histogram shows these counts graphically. The

29

30/ Chapter 4: Enhancing Digital Images

horizontal axis shows, in order, the levels of brightness. The vertical axis showsfrequency-the pixel counts. Vertical bars are drawn for each level of brightness; theheight of each bar shows the number of pixels having that level of brightness. Thefollowing example shows a very small image of 25 pixels having five differentgraytones. The histogram shows, by the height of the bars drawn for each graytone,how many pixels have that graytone.

Frequency

* 5-

U 1

HUEElGraytones

Array of pixels Histogram showing frequencyof each graytone

Typically, air photos and satellite images have many, many more pixels and many morelevels of brightness. Satellite images usually have 256 levels of brightness. The numberof levels of brightness in a scanned air photo depends on the scanner used, andgenerally ranges from 16 to 256.

The example that follows is a subscene (a portion of an image) taken from a scanned airphoto. The scanner used was capable of detecting up to 256 different levels ofbrightness. As in the preceding example, the vertical axis shows frequency. In thiscase, however, the horizontal axis does not show the levels of brightness as graytoneboxes. Instead, it is labelled "Graytone Values" and shows numbers. The mostcommon way of representing levels of brightness in a histogram is to assign a numberto each level. If there are 16 different levels of brightness, the values will range from 0to 15. If there are 256 different levels of brightness, the values will range from 0 to 255.By convention, 0 is assigned to black, with higher and higher numbers assigned tolighter and lighter shades of gray all the way up to white. In this particular example,although the scanner was able to detect 256 levels of brightness, the image didn't havethat many. The lowest value is 6 (rather than 0) and the highest value is 207 (ratherthan 255).


Frequency

40

- '30

I III Ii 1.1 l1lb,- --- --

- ~20ii

- 10

-~ 0

6 56 107 157 207Graytone Values

A scanned image of a building and roads. Histogram showing the frequency of eachThe image is enlarged to show the pixels. value (i.e., each graytone) in the image.

What enhancement techniques are available? Enhancement techniques vary widelyfrom one image processing package to another and can include controls for brightnessand contrast, different filtering techniques, and a family of techniques called contraststretching. Two techniques that are widely available and useful, brightness andcontrast control and contrast stretching, are described here in general terms.

Brightness is a measure of the lightness or darkness of an image. When brightness ishigher, the entire image is lighter; when brightness is lower, the entire image is darker.Contrast is a measure of how the different graytones are distributed in an image. Whencontrast is higher, there is a greater range of graytones, from very dark to very light.When contrast is lower, there is a narrower range of graytones.

Brightness and contrast can be controlled during scanning (as discussed in chapter 1).These controls can also be applied to an existing image. This chapter shows the resultsof applying different enhancements to existing images using image processing software.Each example consists of a subscene taken from satellite data, along with the histogramshowing the distribution of graytones in that image. Comparing the visual appearanceof the image with the accompanying histogram can help clarify what the termsbrightness and contrast mean in image enhancement.


The following example shows a subscene taken from satellite data. The histogramshows the distribution of brightness levels and confirms what simply looking at theimage shows: the image is relatively dark and low in contrast, with all of the valuesclustered within a narrow interval in the dark end of the range of possible values.

Frequency

1.0 Values 2554 Darker Lighter -0

Original image. All the values are clustered ina relatively narrow interval in the darker end ofthe graytone range.

If the brightness of the image is decreased, the image becomes yet darker. Thehistogram looks as if it were pushed to the left-all the values have become lower andall the pixels have become darker.

Frequency

0 Values 2554 Darker Lighter -0'

Decreased brightness. The shape and size of thehistogram are unchanged and all the values are shiftedto the lower-valued, darker end of the graytone range.


If the brightness of the image is increased, the opposite happens: the image becomeslighter and the histogram is pushed to the right.

Frequency

0 Values 2554 Darker Lighter --

Increased brightness. The shape and size of thehistogram are unchanged and all the values are shiftedto the higher-valued, lighter end of the graytone range.

If the contrast of the image is decreased, there is less variation in the levels of brightnessin the image. As the histogram shows, the narrow range of values becomes stillnarrower.

Frequency

0 Values 255'0-Darker Lighter -

Decreased contrast. Notice how all the values areclustered in a narrow graytone range.


If the contrast of the image is increased, however, there is greater variation in thebrightness levels of the image, and the histogram is spread out over a greater range. Inthis case, increasing the contrast of the image is an improvement, making it easier todistinguish features.

Frequency

I 11ll1lllllll]I 1]]] Igliiiii.....1O Values 2554- Darker Lighter -

Increased contrast. Notice how the values arespread out over a greater graytone range.

Often brightness and contrast are best used together. In this case, increasing both thebrightness and the contrast of a dark, low-contrast image produces a better image. Theriver stands out more clearly and the terrain around the river is much more distinct.

Frequency

0 Values 255Darker Lighter -

-- A combination of brightness and contrast adjustments.


Contrast stretching is a family of techniques used for contrast control. Most imageenhancement programs do simple contrast control by allowing the user to specifyhigher or lower contrast without reference to a histogram. Contrast stretching allowsmore control. The user can specify the range of graytones used to display and print theimage, the mathematical model used to perform the stretch, and the way in whichvalues in the extreme light and dark ends of the range are treated.

Contrast stretching generally takes advantage of the full range of graytones possiblewith the computer, screen, and printer used to process, display, and print the image. Acomputer may have 256 or more levels of gray values available for processing an imagenumerically. Screens and printers frequently have 32 levels of gray, and an image willoften use only part of this display range. In a contrast stretch, the highest value pixelsbecome white, the lowest value pixels become black, and the in-between values arespread out across all the possible grays between the two extremes.

There are many different ways to stretch the mid-range values. Some make use ofmathematical models, some depend on the judgment of the user, and some use acombination of both. The examples shown here use a specific type of contrast stretchcalled a linear contrast stretch . The linear contrast stretch is probably the mostcommonly used form of the technique.

The following example shows a satellite image of a river valley from the same area asthe examples on pages 32 - 34. As before, the original image is both dark and low incontrast.

Frequency

Original image. The valuesare clustered in a narrow range,producing a low contrast image.

0 Values 2554 Darker Lighter --


When a contrast stretch is applied to this image, it becomes easier to see features such assand bars, a steep cliff on the north bank of the river, and the surrounding terrain. Thehistogram shows how the original graytone values have been redistributed. Thelowest-value pixels have become black, the highest-value pixels have become white,and all of the pixels in between have been reassigned shades of gray across the entire

range of grays.

iiFrequency

Contrast stretch. The display values have beenstretched out to use the entire range of brightnessvalues.

'I ..i. .. . .11111 lii i.. ............0 Values 2554 Darker Lighter -

The next example shows a more extreme contrast stretch. In this case, only the middleportion of the histogram is stretched across the range of graytones. Cut-off points havebeen selected at each end of the distribution. Any pixel with a value greater than orequal to the upper end cut-off point becomes white in the enhanced image; any pixelwith a value less than or equal to the lower end cut-off point becomes black in theenhanced image. The remaining pixels, those between the two cut-off points, areassigned graytones stretched across the range of grays. Different cut-off points werechosen experimentally and the results were evaluated. With experience it becomeseasier to choose these cut-off points, but the deciding factor should always be thequality of the results.

In this image, some areas have become washed out, but other areas, in particular thosenorth of the river, show much more detail. This specific contrast stretch is not the bestoverall enhancement, but it is good for a subarea of the image.


-* a-

-"W

; / ' u- v )\--e^

- + K-t >

Frequency An "extreme" contrast stretch. Values in the tailsof the histogram were grouped together; the remainingvalues were emphasized by stretching them over thefull range of brightness values.

0 Values 255Darker Lighter --

All pixels with values ^ All pixels with valuesbelow this cut-off I above this cut-offpoint become black. j point become white.

Darker - Lighter

Pixels with values in the mid-rangeare assigned graytones stretched outover the entire possible range.

The results of this extreme contrast stretch demonstrate a common result of imageenhancement: an enhancement that improves an image in some regions may actuallydegrade it in others. It may make sense to create more than one display version of animage to bring out details in different regions. As usual, the intended use of the imageis very important. If an image is needed to provide information about agriculturalareas, an enhancement that brings out detail in cultivated fields at the expense of urbanareas is a good enhancement. If, however, the image is used to look at settlementpatterns, a different enhancement would be desirable.

Chapter 5Digital Mosaics

What is a digital mosaic? A digital mosaic is a singleimage file formed by joining together two or moresmaller image files. Digital mosaics can be formed byjoining together individual scanned air photos orportions of large scanned maps or images. The term

n ? mosaic is used because, like a decorative mosaic (a____ design formed by piecing together small bits of stone

or glass), a useful whole is formed by putting togethersmaller pieces.

, = .Sometimes photos or maps are much larger than thescanner window and must be scanned in two or more

pieces. These pieces can then be combined digitally to form a mosaic-a digital imagecorresponding to the large original.

Air photos are almost always taken in sets that cover the area of interest, such as a city,a river valley, or some other landscape. When the photos are scanned, the result is a setof image files. To get a single, complete picture, or "photomap," of the area of interest,the image files need to be joined together into a mosaic.

Why are digital mosaics useful? There are two types of digital mosaics discussed here.The first is formed by joining together the scanned pieces of a large air photo or map. Inthis case, the result is a digital file that shows, in one digital piece, everything that wasin the original. It is easier to work with this one file, whether viewing, processing, orprinting, than with several files. The second type of digital mosaic is formed by joiningtogether two or more air photos from a set over a given area. The result is a seamlesscomposite image, showing a larger area than any of the individual air photos.

A very important use of digital mosaics is as a source of subscenes. A slubscene is a partof a larger image-for example, an image of one particular neighborhood within a city.If an image of this neighborhood is needed, display the digital mosaic showing the city,select the area showing the neighborhood, and copy and save the subscene as anindependent image file.

Subscenes are selected from a mosaic without regard for the boundaries between theoriginal images and can be made up of parts of two or more scanned photos. Subscenesof different shapes and sizes can be selected repeatedly from the master image as theyare needed. Because the subscenes are copied from the original image, there is nochange to the master image, which can be used again and again.

39

40/ Chapter 5: Digital Mosaics

The white box on the satellite image above shows the approximate location of the digital mosaic

of scanned air photos shown below. Close inspection of the mosaic will show boundaries

between individual photos as well as notations made on the photos before scanning.

'71. -l

.-- i


How are digital mosaics formed? Depending on the type of image files, different typesof software can be used to form digital mosaics. Two techniques are described in thisbook, one using general-purpose drawing software (chapter 6) and one using imageprocessing software (chapter 7). Many people find building a mosaic with drawingsoftware to be a more intuitive process and one that is easier to learn. Drawingsoftware also has the ability to rotate scanned images, which is useful in correctingrotation that occurred during scanning, either purposely-to fit the map or photo intothe scanning window-or inadvertently, because of imperfect alignment. On the otherhand, some drawing programs cannot work with graytone images and so cannot beused to create mosaics of satellite images, scanned air photos, or maps scanned ingrayscale mode. When a drawing program with graytone capabilities is not available,image processing software can be used to build a digital mosaic. In either case, theseprograms must be able to handle the very large files that almost always result fromcreating a digital mosaic.

What sorts of limitations are there with these techniques? Because the techniquespresented here are basic and simple, they also have limitations. For example, whencreating a mosaic of scanned air photos, these techniques do little or nothing to correctfor distortions inherent in the photos or to reconcile visual variations between photos.

All unprocessed air photos have a certain amount of distortion due to, among otherthings, the camera lens. Photos tend to be most accurate at the center of the image andincreasingly distorted toward the edges, often making it impossible to get a perfectalignment of adjacent photos. The effect of distortion in a digital mosaic can beminimized by including only the middle, least distorted, part of the photos in themosaic. Air photos are always taken so that there is overlap between adjoining photos.This is done to ensure continuity between the photos, or, in many cases, so that thephotos can be used for stereo viewing. (In stereo viewing, two photos of the same area,taken from slightly different vantage points, are viewed simultaneously, usually using adevice called a stereoscope. When viewed this way, the landscape appears three-dimensional.) The greatest distortion occurs at the edges of the overlap. The edges canbe eliminated either during scanning by selecting the middle of each photo as thescanning area and leaving only the minimum overlap necessary for aligning adjacentphotos, or by cropping the image files after scanning.

Careful, selective scanning or after-the-fact cropping will reduce the amount ofdistortion but will not eliminate it totally. If greater accuracy is required, corrected airphotos, referred to as geometrically rectified photographs or orthophotos, can often bepurchased from commercial firms. Orthophotos are more costly than are uncorrectedair photos, but they provide a level of accuracy that cannot be obtained withoutspecialized resources and expertise.

42/ Chapter 5: Digital Mosaics

Another issue that is not addressed here is global digital enhancement of a mosaic.When global enhancement is applied, the brightness levels of adjacent photos arematched before building a mosaic. This results in a mosaic without noticeablebrightness variations across photo boundaries. For some uses, such as trackingdesertification, this type of enhancement strategy is best. However, it is also moredifficult and time-consuming than local enhancement on a photo by photo basis.

When local enhancement is used, each image is enhanced individually, without regardfor what is done with the other images making up the mosaic. It is relatively quick,easy, and inexpensive and gives control over what types of features are optimized ineach image. However, a mosaic formed of scanned images that have been locallyenhanced can have a patchwork appearance, with adjacent photos having visiblydifferent levels of brightness.

It is clear that the techniques presented here for creating digital mosaics havelimitations. However, they provide a basic approach that can either be refined and builtupon, or which, in many cases, yield a less-than-perfect but quite usable result.

Chapter 6Technique-Creating a Digital MosaicUsing Drawing Software

- ' When a single large map or image has been scanned in more. -than one pass, or when a set of air photos covering an area

has been scanned, the result is a set of image files. Theseifiles are generally more useful when they have beencombined into a digital mosaic. In the first case, the mosaic

.-. . is a single file corresponding to the original map or image.In the second case, the mosaic is a single seamless imagecovering the area of interest.

When image files are the result of scanning a map in line art. 7mode, using drawing software is probably the best way to

create a digital mosaic. In some cases, drawing software canalso be used to create a mosaic from grayscale images. Todo this, two conditions must be met: the drawing softwaremust be able to display, work with, and save grayscale filesand it must also be able to deal with the very large files that

The Hardware often result from scanning in grayscale mode.

A personal computer that has adequate RAM.

Personal computer A removable mass storage device that is capable ofrecording the large files resulting from creating a digital

Removable mosaic. The device should be usable for both on-line (i.e.,mass storage device working) storage and for backup.

The SoftwareI kXSystem software. This is the basic software that makes the

computer run and allows other software packages to beused. It is contained on the computer's start-up disk.

System software[ L<Drawing software. This software displays the scanned

maps or images and joins them together. It can also beused for updating, editing, and printing the maps once the

Drawing software digital mosaic has been formed.

43

44/ Chapter 6: Technique-Creating a Digital Mosaic Using Drawing Software

Creating a Digital Mosaic UsingDrawing Software

Import the individualimage files to adrawing program.

Create the drawing

(2) A file; make itbig alenough to hold allthe scanned images.

Paste in other3 ) scanned images,align manually.

L---_


The process begins with importing the scanned files to the drawing program. Next, adrawing that will become the mosaic file must be created. All the pieces of the map orimage will be pasted onto this drawing, which must be big enough to accomodate themall. Then, each piece of the map or image is brought in, pasted onto the mosaic, andaligned manually. The result is a digital mosaic.

To begin the process, thescanned files must be

i -imported into the drawingImport the individual program. The most common

1 image files to a porm h otcmo( )method of importing a file isdrawing program. to open the file from the

drawing software. Often, amessage will be displayedstating that the file is being

translated. Once the file is opened, it can be viewed, edited, and copied. It can beclosed without saving, in which case it reverts to its original format, or it can be savedin the drawing program's format. All of the scanned files can be imported and thensaved in drawing program format before starting the mosaic-building process. Or, oneat a time, the files can be opened, copied, and added to the mosaic, then closed withoutsaving the translated form. The choice is a matter of personal preference.

The next step is to start themosaic file, a drawing that

Create the drawing Kis big enough toCrle;ate ith draing accommodate all the pieces2 file;hmakehitdbi of the mosaic. When a-enough to hold all scanned file is opened inthe scanned images. the drawing software, it

-- -can be thought of as asnapshot placed on a pieceof blank paper. The

snapshot can be moved around and rotated, text and symbols can be drawn either ontop of the snapshot or on the paper, and the snapshot can be copied and that copypasted elsewhere. In this analogy, the mosaic is built by pasting a copy of each scannedimage (each snapshot) onto a drawing (a piece of blank paper) big enough to hold themall and then carefully aligning them.

To get started, open one image file-usually the one that will be the upper left-handcorner of the finished mosaic. Then, change the drawing size-the size of the blankpaper, not the size of the image or "snapshot"-to make it big enough to hold all theother pieces of the mosaic. Most drawing programs give the size of the drawing in


-I

t.:... .. ..... The beginning of the mosaic. The first pieceof the mosaic is positioned in the upper lefthand corner of the drawing. The drawingitself (visible as grid lines) is large enough for

X ,.. -... ............ .......... ..all of the pieces of the mosaic.

..... . .. E.... .

. . . ... . . . . . . . . . . . . . .

units of the user's choice, such as centimeters or inches. After selecting the drawingsize for the mosaic, save the file using as a file name not the name of the individualscanned file, but the name c-hosen for the mosaic.

The next step is to add the otherpieces of the mosaic, one piece ata time. Each image file is

Paste in other . opened and displayed, and the3 scanned images, -drawing program's copy.x align manually. -funmction is used to copy the

scanned image. Then, the

z fmosaic file is opened and thecopied image pasted onto it.Finally, the scanned image is

aligned with the part of the mosaic already in place by moving it on the screen usingscreen tools. This is often referred to as manual alignment because it parallels the processof aligning air photos or pieces of a paper map by hand.

Some drawing programs offer a place function that replaces the copy and pastesequence. When this function is available, the mosaic file stays open and each scannedimage is placed in it by selecting the place function and naming the desired image file,without having to open that file. The end result is the same: one file containing all theparts of the mosaic.


* ,Ewp

S A

-__- ----

- rf ,-*i

1 1

When drawing software is used to create a digital mosaic of scanned air photos, thescanned images are brought in to the growing mosaic one by one and aligned manually.

ft. ''

C 4AS ^ ''.,


, . -. 1Yi ^' , .....

The same growing mosaic shown at different display scales.

V I)CALJDF. KE MATOLA .0 iVI

14541 IIABtAlFTES TANTES ,- If

i* 4- * .,r., bf 'f 4

In aligning the pieces of the mosaic, different display scales are used at different points

in the process. The display scale does not change the image file itself, only the way it

appears on the screen. Displaying the growing mosaic at a reduced scale provides an

overview of the entire mosaic. A larger scale shows detail, which helps when doing

precise alignment.

The easiest way to align two images is to focus on adistinctive feature such as a road or large building, or :

on a piece of text. Once the feature or text has beensuperimposed, change the scale to get an overviewthat will show whether further adjustments areneeded. As each piece is added and aligned, save the RITO VI ) VImosaic to minimize the chance of losing the growing HABITANTES .'.NT. S

mosaic. ITANTE5

Then, the process of opening an image file, copying, Enlarging the displayed mosaic

pasting (or placing), and aligning can be repeated is useful when aligning the

until the mosaic is complete. pieces. In this case, the text inthe overlap region can be used toline up the map pieces precisely.


Alignment involves moving the images horizontally and vertically; it can also involverotating them. Sometimes when portions of a map or image, or individual photos arescanned they are inadvertently rotated with respect to each other. And, sometimeswhen a large map or image is scanned, one or more pieces of it are rotated 900 from theorientation of the rest of the map to fit those portions into the scanning window.Whenever nonstandard orientation is a factor, individual images can be rotated tomatch the orientation of the mosaic.

Sometimes a portion of a map is scannedrotated 900 from the rest of the map. This canhappen when the map portion will fit in themap window when rotated, but not when

!~ . . . . oriented the same way as the rest of the map.In this example, a narrow strip along the bottomof a map has been scanned rotated 900 from therest of the map. The black area in the scannedimage is blank.

When a portion of the map is not oriented the: 1: : :same way as the rest of the map, it must be

rotated before it can be joined to the rest of the.......... ... .... mosaic. Most drawing programs allow the user to

rotate objects either by turning them by hand orby specifying a rotation of a certain number ofdegrees. In this example, the map portion is beingrotated by hand.

Once the map portion has been rotated so that itis oriented the same way as the rest of the map,it can be saved in its new orientation and thenadded to the mosaic.

The finished product can be printed in its entirety or, especially in the case of photos orsatellite images, subscenes can be selected and printed. The mosaic can be used as thebasis for a computer map or linked with other information. Regardless of the intendeduse, the digital mosaic shows all the information contained in all the scanned piecesassembled in a seamless whole.

if

q

ii

Chapter 7Technique-Creating a Digital MosaicUsing Image Processing Software

'V When drawing software capable of handling graytones isnot available, image processing software can be used to

_ build a mosaic of graytone images. A typical example ofN i< this is using image processing software to create a mosaic of

scanned air photos.

Air photos are usually taken as a set to form a picture of aU 'large area such as a city. The photos themselves can beassembled and physically fitted together to form this bigpicture. A similar thing can be done digitally with theimage files created by scanning the photos. The resultingmaster image is almost always very large, both in terms ofits display size and its storage size.

The HardwareX, A personal computer that has adequate RAM and a monitor

that is capable of displaying at least 16 levels of true gray.

Personal computer

A removable mass storage device that is capable ofrecording the large file resulting from creating a digital

Removable mosaic. The device should be usable for both on-line (i.e.,mass storage device working) storage and for backup.

The SoftwareSystem software. This is the basic software that makes thecomputer run and allows other software packages to beused. It is contained on the computer's start-up disk.

System software

I lX 17 Image processing software. This software is used toI E Iprocess images in raster format. Not all image processing

packages have exactly the same capabilities. Care should beImage Processing taken to choose one that can create a digital mosaic.

software

51

52/ Chapter 7: Teclrnique-Creating a Digital Mosaic Using Image Processing Software

Creating a Digital MosaicUsing Image Processing Software

Import the scannedimages to an

1 /image processing ..-

program. l

l 2Find a common point,determine the coordinatesin both images.

r h '

Assign the same( 3 ) coordinates in

both images;join images.


There are three steps in creating a digital mosaic using image processing software.First, import the images into an image processing program with mosaic-creationcapabilities. Second, choose two adjoining images, select a point common to bothimages, and determine the coordinates of this point in each image. Third, change thecoordinates of the point in one image to match the coordinates of that point in the otherimage. This brings the coordinate systems of the two images into agreement-in effect,the two images now share the same coordinate system. This allows the imageprocessing software to combine the two images properly, placing each point accordingto the common coordinate system. The process is continued by comparing images oneat a time with the growing master image, locating a common point, bringingcoordinates in agreement, and joining the images to the master.

Import the image files into theimage processing program.

Import the scanned Image processing programsimages to an generally accept a variety of fileimage processing - formats, but it is important toprogram. be aware of the requirements of

the particular program beingused. One of the most common

_ - _ - -methods of importing a file isto open the file from the software. Sometimes a message will state that a translation istaking place. Once a file has been opened this way, it can be closed without saving, inwhich case it remains in its original format, or it can be saved in the image processingprogram's format.

An intermediate step that is often desirable, though not absolutely necessary, is toenhance the individual images. Chapters 4 and 5 touch on this subject.

- - 1 The mosaic-buildingprocess starts withtwo images that havea common boundary.The choice of whichtwo images to begin

.2 with is a matter ofF - personal preference.

Find a common point, i <The delineation indexdetermine the coordinates that was createdinboth images. 1!when the photos were

scanned (chapter 2,' a "Scanning Air

Photos") can be very

54/ Chapter 7: Technique-Creating a Digital Mosaic Using Image Processing Software

useful at this stage in determining how the images are positioned in relation to oneanother.

5-32 5-34 1 -53 15-58 rc5-3 Scanned Photo5-47 5-60 5-48 5-32

KK-, - Scanned Photo5-55 5-36 5-33 5-34

5-41 5-44 5-49--

Delineation index used to Adjacent images with overlap areachoose images for mosaic

Once the first two images have been chosen, find a common feature in the area ofoverlap. (When the air photos were scanned, the scanning area should have beenchosen to include some overlap between adjoining photos.) Examples of easy-to-identify features include road intersections, distinctive buildings and clearly definednatural features such as coastlines.

r X.

The same feature shown in two different images.

After finding the common feature, determine the coordinates of one pixel in that featurein both images. Coordinates are pairs of numbers used to locate things in a map orimage. Image processing packages use a raster or grid-based representation. Thismeans that maps and images are shown as rectangular arrays of pixels with each pixelrepresenting a parcel of land. The coordinates tell what row and what column eachpixel is located in.


Columns This example shows a very small image made up of0 1 2 3 4 five rows and five columns. The rows and columns

o Pixel (2,3) are numbered beginning at the upper left-handRows 1 comer with zero. The first row is Row 0, the next is

2 Row 1, the next Row 2, and so forth. The columns3 -are numbered in a similar way. Every pixel has a

unique pair of numbers giving its location. Thecoordinates (2,3) tell you that the pixel is found in row 2 and column 3.

The scanned images naturally have many more pixels, but the principle remains thesame. Any pixel in the image can be located precisely with row and columncoordinates. To find these coordinates, zoom in on the pixel in stages. A high displayresolution makes it easy to see the overall scene; as the display resolution becomescoarser, it is easier to see an individual pixel, but more difficult to make sense of theoverall picture.

Zooming in on the same point. Note the trade-off between ease oflocating an individual cell versus seeing the overall picture

A pixelrepresenting thesame point in twoimages generally

.Assignthesame has differenti 3 coordinates in t coordinates in each

- both images; image. To join thejoin images. images together,

the coordinates of.- one of the images

have to be changedS Iso that the pixels in

the overlap regionhave the samecoordinates in bothimages.

56/ Chapter 7: Technique-Creating a Digital Mosaic Using Image Processing Software

(4,7) (4,7)

3 4 506 3 4 5 6O (34,16) ° (4,7)

3 33 3 3

35 5

This diagram shows, in schematic The coordinates of the image on the rightform, two very small adjacent images have been changed so that the commonwith an easily identifiable point point has the same coordinates-(4,7)-common to both shown shaded. hI in both images and the images share aone image, that point has coordinates coordinate system.(4,7) and in the other it hascoordinates (34, 16).

3 4 5 6 7 8 9 100O A mosaic has been formed by2 -------- joining the two images

3-- together using their common4 - coordinate system.567

In the example on page 57, the coordinates of the chosen point in one image are(1630, 1104) and in the other image they are (1624, 551). The coordinates of the secondimage were changed to match those of the first.

Once the coordinates for the two images have been brought into agreement, the imageprocessing software can join the two images together correctly because they share thesame coordinate system. The exact command or commands used to join the two imageswill depend on the software. For example, in one software package the user chooses thecommand "MOSAIC" and names the files containing the images to be joined. Thesoftware then combines the two images into a new one, using coordinates to put themtogether properly.

The rest of the images can be joined to the growing master image using the sametechnique. As the master image grows larger and larger, the joining operation willprobably take longer and longer. This is a natural result of the growing size of theimage file and should not be considered a problem as long as system limits are notreached.


Coordinates: (1630,1104)|*

Coordinates of a cell in the samefeature in two different images.

'N'1

S. * * -

oTwo images joined togetherto form the beginning of a '"master" image. Note thedifference in brightness and a.contrastne in bightness and a

The resulting master image showing the area of interest will probably be too large formost manipulation and will be used mainly as a source of smaller subscenes selected forspecific tasks. The subscenes can be of almost any size and can be located anywherewithin the area, without regard for the boundaries between the original photographs.

Chapter 8Creating a Computer Mapfrom a Scanned Image

s -Maps are drawings showing the location and characteristics ofnatural and manmade features. Although the most familiarmaps are those printed on paper, maps can also exist incomputer form. Computer maps can be created in a number ofdifferent ways, including digitizing existing paper maps with a-r tablet digitizer and using specialized cartographic software.

,5 This chapter and the two that follow introduce a tracingtechnique that can be used to create computer maps fromscanned images using either image processing software orgeneral-purpose drawing software.

What is the difference between an image and a map? Anunprocessed scanned image is a picture made up of pixels.The color or graytone of each pixel is the result of the

reflectance measured by the scanner during scanning. This is true whether the image isthe result of scanning a photo, a satellite image, or a map. A person may look at theimage and recognize a group of pixels as a specific feature; however, that interpretationtakes place in the mind of the viewer-it is not built into the image. There is no

FLLEGEND

-RoadsCommercial/LIndustrial Bldgs]

On the left, an image produced by scanning an air photo and, on the right, a raster map createdfrom the image. In the scanned image, both the roads and the large building in the upper left handcorner can be identified by a person looking at the image, yet the same graytones appear in bothtypes of features. The map shows only some of the features pictured in the image and each type offeature is represented by a single graytone and labelled in the legend.

59

60/ Chapter 8: Creating a Computer Map from a Scanned Image

straightforward way to automatically classify pixels with a certain graytone as oneparticular type of feature.

Maps-including computer maps-are more symbolic. There is a very clear andconsistent correspondence between the shapes and colors or graytones making up amap and the objects and features that they represent. For example, on a road map, aline of a certain color and thickness always represents a main road, while a line of adifferent color and thickness always represents a secondary road. The lines are symbolsof features, rather than just pictures of them.

What types of computer maps are there? Computer maps come in two basic types:raster and vector. Raster maps, like scanned images, are arrays of pixels. Each pixel hasa location (which can be defined using coordinates, as discussed in chapter 7), a visualappearance, and a value. However, while in a scanned image, the values and visualappearance of the pixels indicate reflectance as measured during scanning, in a rastermap, the values have other meanings. For example, in a raster map showing anagricultural area, the value and graytone of each pixel may indicate not reflectance but

the type of crop growing at that location.

Raster images and raster maps also differ in what is shown. The scanning process is notselective. Everything visible in the original image or map shows up in the scannedimage. A raster map, however, is the result of classification and selection. It showsonly relevant features. An image created by scanning an air photo of an agriculturalarea shows everything-the fields, the roads, any buildings, equipment working in thefields, even notations that may have been made on the photos before scanning. A rastermap created from such an image shows only the features that the mapmaker hasselected. For example, it may show only the fields, with values and colors selected tosymbolize the types of crops.

ELEVATION'J3 -~~p-~ 80 m

. } - J - - 120 m

I - 20 m

II ! s280 m

EJ320M

The image on the left was produced by scanning a contour map. The raster map on theright was created from the image by tracing. In the map, noise has been eliminated andmeaningful values have been assigned.


Vector maps use a different representational scheme. While the basic unit of a rastermap is the pixel, the basic unit of the vector map is the object. Objects are symbolsmade up of points, lines, and areal shapes and are used to represent features. The usercan select and process a specific object as an entity, rather than as a collection of pixels.For example, a vector map of an agricultural area would show fields as polygons-arealshapes with sides made up of straight lines-not as groups of pixels. Each polygon (i.e.,each field) could be selected, measured, and edited as a single thing.

A'

T'he irnage on the left was produced by scanning a hand-drawn map. On the right is thecomputer map created by tracing that irnage. The map is in vector forrnat-districts,neighborhoods, and buildings are represented by polygons.

How does tracing create a useful map? Both raster and vector maps can be createdfrom a scanned image by screen tracing, a technique very similar to manual tracing. Ascanned image is displayed on the screen, and features are traced using a computermouse and screen drawing tools, such as line- and polygon-drawing tools. The processis similar to placing a transparency over a paper map or image and tracing featureswith a pencil. Depending on the type of software used and the type of map desired, theresult of tracing is either a raster or a vector map.

Tracing performs several functions. It is used to extract specific information, leavingbehind other information and noise. Meaningful values can be assigned to features aspart of the tracing process. Tracing is also a way to create a vector map based on araster image. Although spatial data in raster format is our primary focus, there aremany times when a computer map in vector format is more useful. Tracing is one wayof moving from one representation to the other.

62/ Chapter 8: Creating a Computer Map from a Scanned Image

The following two chapters illustrate two types of tracing. In the first method, scannedimages are traced using image processing software, resulting in maps in raster format.In the second, a scanned map is traced using drawing software, creating a digital mapin vector format.

Rastertra[ig prcessComputer maps

tracig proessn

-Raster format

Paper maps and images

Scan

Computer maps-Vector m

tracing process Vector format

-e .

Chapter 9Technique-Creating a Computer Mapin Raster Format

Scanned images are computer pictures. They can bepictures of (among other things) maps or air photos. Screen

v , tracing is a method of using scanned images to create4 computer maps that represent features in an explicitly

symbolic way. The process does not destroy or alter theoriginal images, which can then be used for other things.

When tracing is done with image processing software, it canproduce a map in raster format-a map that is an array ofpixels in which the graytones represent not scannerreflectance but specific features.

The HardwareK -A personal computer with adequate RAM and a monitorl that is capable of displaying at least 16 levels of true gray.

Personal computerA removable mass storage device that is capable of storing

L__;= the large files that are often involved in working withRemovable scanned images The device should be usable for both on-

mass storage device line (i.e., working) storage and for backup.

The Software System software. This is the basic software that makes the__ Lcomputer run and allows other software packages to be

used. It is contained on the computer's start-up disk.

System software

Image processing software. This software can processimages in raster format. Not all image processing packageshave exactly the same capabilities. To create a map from a

Image Processing scanned image, the software should be able to display asoftware scanned image and mark it using drawing-type tools.

63

64/ Chapter 9: Technique-Creating a Computer Map in Raster Format

Creating a Computer Mapin Raster Format

( 1 8 Import scanned image fileto image processing software.

2 aTrace features usingdrawing tools.Assign initial values.

Discard everything

( 3 0 except3 raced featuressf

Assign useful ELEVATION

colors and 80 mvalues. 120 m

Create legend. 160 m

Annotate.= 200 m

-r4 240 m

il 280 m

i 320 m

360 m


As shown on page 64, a map can be created from a scanned image in four steps. First,import the scanned image to the image processing software. Second, trace the featuresof interest using the program's drawing tools. Third, discard or hide everything in theimage except for the traced features. Fourth, complete the map by assigning usefulgraytones and values to the traced features, adding annotation as necessary andcreating a legend.

i-- The map creation process can beapplied to an individual scanned1 mport scanned image file image or to a digital mosaic. In

to image processing software. -- either case, it may be useful toenhance the image before tracing.Digital image enhancement andthe creation of a digital mosaicare discussed in chapters 4through 7.

If the image is a digital mosaic or if it has been enhanced, then it may already be in aformat that can be used by the image processing program. If neither of these steps hasbeen taken, then use the image processing program to open the scanned image. This, ineffect, imports the scanned image. Once the image file has been opened, it can bedisplayed and manipulated using the image processing software. It can also be savedin the format used by the image processing software.

Once the image has been imported,L Stracing can begin. This is done in' ' Trace features using different ways depending on the( 2 ) drawing tools. F software used. Some software packages

J Assign initial values. allow the creation of layers. In this case,the map file can be thought of as a stack

I_ of transparencies. The scanned image isthe bottom layer, with one or more

blank, transparent layers created on top of it. Features are drawn on the blank layerswith a computer mouse and drawing tools, tracing the image below. All tracing can bedone on one layer, or a number of layers can be used, with each layer showing a singletheme. For example, an image of an urban area might be used to create a map with onelayer for municipal boundaries, another layer for streets, a third layer for buildings, afourth layer for parks and open space, and so forth.

Other software packages do not use layers, in which case tracing is done directly on theimage. With this method, it is especially important to make a copy of the originalscanned image. One copy is kept as a back-up and reference, and the actual tracing is


done on the other copy. As with the layering method, all tracing can be done on asingle image, or themes can be separated. If themes are separated, a copy of the imagemust be made for each theme.

Scanned image Tracing in layers. Each type of feature eachtheme-is traced as a separate layer. Taken

together the layers form a thematic map.Coastline layer Layers can be edited and used

individually or in combination.

5 hsizRoads layer

Buildings layer Thematic map

Whether using layers or the image itself, tracing is done with a mouse and screendrawing tools. These tools are used to draw lines, polygons, and freehand or irregularshapes. (Some programs include other tools, but these are the most common.) Themost useful tool is generally the polygon tool. It can be used to draw both closed andopen polygons. In this context, polygon has a slightly different meaning than in day-to-day usage. Polygons can be either closed or open. A closed polygon is an areal shapebounded by straight line segments. Closed polygons can represent features such asbuildings, agricultural fields, airplane landing strips, or bodies of water. An openpolygon is a connected sequence of line segments. Open polygons can representfeatures such as roads, boundaries, contour lines, or rivers.

Examples of closed polygons. Examples of open polygons.


Before actually tracing, display the image at a relatively small scale to get a goodoverview and then experiment with larger and larger display scales until a comfortableone is found.

-t It - k

Increasing display scale to zoom -r%O'

in on one feature for tracing.

Most programs allow the user to choose the color or graytone assigned to a drawingtool; it is best to choose something that contrasts clearly with the image being traced sothat it is easy to see what has been traced and what has not.

P,;44 4 l--,,-

Ontelf, A bidn a bee trce on a -scne aihtsacoeoyo. On th rgta

\LZ. a X>,~~ ~~ . a.X

contour line has been traced on a scanned map as an open polygon. In both cases, the tracing graytonewas chosen to contrast with the image and the tracing tool is shown as a white cross.

Usually, the values associated with pixels that are drawn with the drawing tools canalso be chosen. The choice of values used at this stage of the process is important. Theyshould be different from any of the values assigned to pixels in the original scannedimage in order to simplify the step of removing or hiding everything except for tracedfeatures. Values should also be selected to allow identification of features. Forexample, if buildings are being traced and no distinction is necessary between types ofbuildings, then all buildings should be traced using the same value.


A scanned image with contour lines traced Graytones Values

directly on the image. All the contour lines 86are black so that they contrast with thescanned image and are easily visible. Each 1 29

traced contour has been assigned a value. In 172

this case, each traced contour has a value Z 247

equal to the elevation plus 1000. This means 248 R

that any pixel with a value greater than 1000 Reflectance values

is a traced contour and any pixel with a value 249 ranging from

less than 1000 is the result of scanning. 250 86 to 254.

251

252

253

25

1080 Values assigned to

1 120 traced contours.Each traced

1 160 contour has value

1200 equal to elevation

1240 plus 1000.

1280

132

Once tracing is done, everything exceptthe traced features should be removed

Discard everything from the map. If the map file has been

3 traced features. created using layers, this is usually done- ----- ; by either hiding or discarding the image

- >layer. If the map has been created by_ _-- -tracing on top of the scanned image, the

process usually involves performing anoperation with the pixel values. Generally, the values of the pixels remaining from theoriginal image will all be in one range and the values assigned to the traced featureswill all be in a different range. The software can be directed to set the values of all thepixels in the first range equal to a value meaning "background." The result will be amap showing the traced features and, where the image originally showed through, onlybackground.


w

* .. ^-

The scanned image. Traced features with the Traced features with thescanned image showing scanned image hidden orthrough. removed.

The final step is to perform a variety of

Assign useful ELEVATION cleanup tasks: using drawing tools to makecolors and 80 m small changes to the traced features; assigningvalues. 120 m colors and values to the traced features; and,Create legend. 160 m when the software allows, creating a legendAnnotate. 200 m and adding text, such as a map title.

_-_ 240 mn Sometimes image processing capabilities can240 m be used to simplify some of these steps.

- [ 320 m Once the background image has been-360 m removed from the map, it is easier to see the

- _ - - - traced features clearly. At this point, it mightbe desirable to do some editing. For example,

a boundary line may be discontinuous, or a building or playing field may be shownwith crooked sides. This type of minor change-making the boundary continuous orthe side of the building straight-can be done with the same drawing tools that wereused to trace. When editing a map, it is very useful to have the original image, eitherprinted or displayed on the screen, available for reference.

F r-

A minor correction was madeto a traced feature after thescanned image was removedor hidden.

i j k


Colors and values can also be assigned to the traced features at this time. The mainconsideration is usefulness: what colors will make the information in the map visuallyavailable and work best when printing the map; what values will be most useful whenmanipulating the map or combining it with other information?

When possible, create a legend. A legend lists and labels or explains the differentsymbols-the different colors and values assigned to pixels and the different shapesformed by the pixels-used in a map. Text such as titles and labels can also helpcomplete the map.

lELEVATION ABOVEGraytones Values SEA LEVEL

-1080 80 meters-1120 120 meters110 160- meters

- 200 meters

-1240 240meters1280 280 meters

1320 320 meters

The scanned image has been removed-all pixels Useful graytones have been assigned to thewith values less than 1000 have been "blanked contours. Values have also been reassigned toout" leaving just the contour lines. equal the elevation. The legend shows the

meaning of each graytone.

The resulting map will differ from the original scanned image in one very importantway: information that was pictured in the image is, in the map, selected andrepresented in a symbolic way that makes it more accessible and more useful. This mapcan be edited, updated, combined with other information, and printed.

Chapter 10Technique-Creating a Computer Mapin Vector Format

When a map is scanned, the result is an image file-aE , * ~computer picture of the map. Although the image can be

T )[ stored and printed, providing a way to get multiple printedcopies of the map, the image itself is not truly a computermap. The image must be processed before it can be used asa computer map. This processing can be done using

. \ // E specialized software; it can also, in many cases, be doneo \ using general-purpose drawing software. The drawing

software is used to trace the features pictured in the image,creating a computer map in vector format.

The Hardware

A personal computer with adequate RAM and a monitorthat is capable of displaying at least 16 levels of true gray.

Personal computer

A removable mass storage device that is capable of storingthe large files that are often involved in working with

Removable scanned images. The device should be usable for both on-mass storage device line (i.e., working) storage and for backup.

The Software

I[1 System software. This is the basic software that makes theLI computer run and allows other software packages to be

used. It is contained on the computer's start-up disk.System software

__ Drawing software to display the scanned maps and tracethe features pictured in them. This software can also beused for updating, editing, and printing the maps once the

Drawing software digital mosaic has been formed.

71

72/ Chapter 10: Technique-Creating a Computer Map in Vector Format

Creating a Computer Mapin Vector Format

Importscanned imageto drawing software.

2 Create thematiclayers.

Boundaries o

Blocks 0

Dwellings

Layer by layer, trace -V3 features using the

polygon tool.

Bairroda Salina

Assign colors l C :

4 Annotate. D


As shown on page 72, a map can be created from a scanned image in four steps. First,import the scanned image to the drawing software. Second, create thematic layers.Third, trace the features of interest using the program's drawing tools. Fourth,complete the map by assigning useful graytones and values to the traced features andadding annotation as necessary.

r _ Either an individual scanned image or adigital mosaic can be traced. If the

r s e image is a digital mosaic, then itmto drawing software probably is in a format that can be used

by the drawing software. If not, theimage can be imported by activating

---- - the drawing program and using it to' open the scanned image. A warning

message may be displayed, alerting theuser to differences between the format of the file being opened, and the program'sstandard format. Once the image file has been opened, it can be displayed andmanipulated using the drawing software. It can also be saved in the format used by thedrawing software.

-0_______ - - ___ '__ - _- - k

2 reae tematic -

Boundaries

Blocks

Dwellings

Most drawing programs make it possible to work with more than one layer within asingle drawing. These layers can be thought of as transparencies placed on top of eachother, with the scanned image at the bottom. When creating a map, it is useful to haveone layer for each theme, or type of information, such as roads, boundaries, ordwellings. Using thematic layers has several advantages. For instance, during thetracing process, it is possible to concentrate on one type of feature at a time. Once thecomputer map has been created, different combinations of layers can be selected for on-screen viewing and printing to suit different uses. The thematic layers also simplify


updating because, again, only the type of feature being updated need be displayed. Thenumber and type of layers created depend on the type of map being traced and the usesto which it will be put.

Before any tracing is done, the thematic layers that are needed should be identified andcreated. When this is done, the map file will consist of the scanned image with one ormore transparent layers on top of it.

_- Tracing is done one layer at a timeusing drawing tools. The scannedimage is displayed on the bottom

3 Layer by layer, trace - and the layer being worked on isfeatures using the activated and put on top. Usually,polygon tool. one thematic layer is completed

- M5before moving on to the next.OM --- Completed layers can be either

hidden or displayed whileworking on successive layers, depending on what proves most helpful.

Drawing programs offer a variety of drawing tools to draw straight and freehand linesand both regular and irregular shapes. The most useful tool for tracing maps is usuallythe polygon tool. Polygons can be either open or closed. Open polygons are a sequenceof line segments. Closed polygons are shapes, either regular or irregular, with straightsides. Linear features such as roads, boundaries, and rivers can be traced as openpolygons. Areal features such as buildings, districts, and bodies of water can be tracedas closed polygons.

There are several techniques that can be used to make tracing easier. Drawingprograms usually allow different display scales. Use a small display scale to get anoverview; use a large display scale for tracing all but the largest features.

Both the thickness of the polygon outline or boundary and the pattern used to fill it canbe chosen. During the tracing process, choose outlines and fills to contrast with the

w-IClosed polygon, Closed polygon, Closed polygon, Open polygon, Open polygon,transparent fill patterned fill transparent outline, thin black outline wide gray outline

graytone fill


background image. It should be possible to look at the displayed image and easilydistinguish between features that have been traced and the background image. Thegraytone and thickness of outlines as well as the graytone and pattem used for fill canall be changed after tracing is completed.

This map file is made up offour layers: the scanned image don the "bottom", then threethematic layers showingboundaries, blocks, anddwellings. Tracing is done onelayer at a time.

Scanned Image

0

0 a 0 D

Boundary Layer Block Layer Dwelling Layer

Once the tracing is completed,Bairro hide the scanned layer. One layerda Salina k at a time, choose graytones, fill

l- patterns, and outline width for theAssign colors traced features. Add labels andaI- and values.4 Annotate. Io other text as necessary. Create one

last layer for the map title and, / = \ legend, which shows the meaning

of each symbol used in the map.


The result is a computer map in vector format. Unlike the original scanned image,which was in raster format, the various features shown in the map can be treated asobjects. Copies of this computer map can be printed quickly and cheaply, at differentscales and with different combinations of thematic layers. The computer map can alsobe updated relatively quickly and easily, to make either corrections or additions.

Bairro da Salina 4(

Legend

A Section Boundaries

4 Blocks 1 2 C |D Dwellings

As a final step, the map of the Salina neighborhood has been annotated. Labels identifying thesections of the neighborhood and the blocks within it have been added to the map. A north arrowshowing the map orientation, a legend defining the symbols used in the map, and a title for the maphave also been added. Annotation can be done on a separate layer.

Chapter 11Marking Maps and Images

What is meant by marking? The most common way to usemaps and images is simply to look at them to obtain orcommunicate information. The next most common way is tomark them, to add information by drawing symbols and text onexisting maps and images. Marking is generally done by theperson using the map or image rather than by the originalmapmaker, although sometimes these two people are the same.

* -6' yPij v4 w - .v1 Why is marking important? Marking is a very important partla '4< % of updating and correcting maps. It is also used in groundflji validation-confirming or disproving feature classifications

i- dmade from air photos or satellite data. Marking can also be the

V W - ---- --------

.. m

viua reernc an manual marking.

"a_ , 7-

§.;<-~~ ~ ~ .C,g P /s;

'** ^' 5 v4

- %. Ch

Toogahia shet wer scnnd moace,poesd nrned, hn ae to th il oviua reeec an maua marking.

,\ tJIw ,,,w w77

78/ Chapter 11: Marking Maps and Images

basis for more advanced map use such as measuring the features shown in maps andimages.

Marking is important because both map making and map use are dynamic processes.Maps and images represent the world around us, which is constantly changing. To betruly useful, maps must keep pace with change. Often, the most common change is in

man-made features such as roads and buildings, but natural features such as rivers and

coastlines change, too. The way maps and images are made and used changes as well.A map that began as a simple city street map may, with the addition of otherinformation, be used to plan the delivery of services such as water or electricity.Modifying an existing map or image, whether to show changes or to adapt it to adifferent use, almost always begins with marking.

How is marking done? A paper map or image is marked by drawing on it with a toolsuch as a pencil or a felt-tipped marker. A digital map or image is marked by usingsoftware drawing tools. Often, the information on a marked hard copy is, at somepoint, transferred to a digital version of the same map or image. Digital marking mayalso be based on other sources, such as different maps or information recorded as textor numbers.

The graphic elements used in marking are the same as the graphic elements used for

map making: points, lines, and areas. Points show things such as wells, water meters,or latrines. Lines represent linear features such as water pipelines or administrativeboundaries. Area symbols represent neighborhoods, forest patches, land-usecategories, and similar features.

k a

.4

Marking a paper image-map.


Annotation, both as text and symbols, is another form of marking. Examples ofannotation include map titles, labels identifying features, a legend to display and definethe symbols used in a map, a north arrow to show the compass orientation of the map, ascale to define the distances shown in the map, text comments, and, in some cases, areference grid. Any or all of these types of annotation may have been present in theoriginal map; the marking stage can include both updating and correcting existingannotation or adding it as needed.

What are some marking considerations? There are several practical considerations thatcan make map and image marking easier. When possible, laminating paper maps andimages -nclosing them in a thin, transparent plastic film-is very useful. Laminatedmaps are very durable, making them attractive for field use. The plastic surface can bemarked, erased, and remarkedagain and again, allowing

-corrections and repeated use. It isalso useful to have multiple copies ;of a map or image that is being 4smarked. One copy should alwaysbe kept for reference; the otherscan be used by the marker. A-;

When marking, paper and digitalversions of the same map or imagecan be used together veryeffectively. A digital map orimage can be used to producemultiple paper copies at anappropriate scale. After the hardcopy has been marked, the processof transferring the marking back to Marking a digital map using a portable computer.the digital form is simplifiedbecause there is an exact correspondence between what is seen on paper and what isseen on the computer screen. The transferral process can be made even easier when thehard copy is printed with rulers or with a reference grid, making it relatively simple tobring up on the computer screen the region to be marked.

The paper maps are cheap, easy to carry into the field and use there, and both readingand marking them are familiar processes for many people. Computer maps are veryflexible, lend themselves well to editing and updating as well as more advanced mapuse, and provide a very practical means of producing up-to-date paper maps. Markingis a link between the two types of map.

80/ Chapter 11: Marking Maps and Images

-, -

f A} a_

Digital satellite data was acquired. Planned settlements were marked by hand on

Both planned and spontaneous settlements, initally marked by hand on atransparency, were transferred to a digital version of the satellite image. Aprinted, annotated version of this image/map is used as an example in chapter13, Hard Copy Output: Printing Maps and Images."

Chapter 12Measuring Maps and Images

What is measured in maps and images? Maps and images arespatial representations of the world around us. One importantuse of these representations is to measure the features shown inthem-for example, calculating the distance between two points;finding the length of a linear feature such as a railroad or

- - pipeline; measuring size such as the area of a body of water orffthe length of its perimeter; and determining buffers or corridors

such as distances from a water source or a road. There are alsomeasurements that are associated not with specific features, but

|- with the map or image as a whole. These include scale-theratio between distance as it is represented in the map or imageand as it exists in the real world-and the orientation, which

i-- I relates the map or image to compass directions. Finally, thereare many types of measurements that are more sophisticated but

less frequently used, including lengths and areas of nonflat surfaces and volumes, butthese are not covered in this publication.

The types of measurements discussed here can be made with paper maps and images,usually with the aid of pencils and rulers. They can also be measured on computermaps using software. Both the type of map representation-whether raster or vector-and the specific software used will affect the ease with which certain types ofmeasurements can be made and the accuracy of those measurements.

Some errors are built into any spatial representation, whether it is a map or an image.Both maps and images are two-dimensional representations of a three-dimensionalworld, and because of this, certain compromises-and hence, inaccuracies-areincluded in them. Further, errors can occur at any point in map making and imageprocessing: in acquiring air photo or satellite data, in scanning maps and photos, and inprocessing digital data. Given enough time, money, equipment, and trained personnel,many of these errors can be detected and corrected. Without these resources,uncorrected maps and images can still be useful and can still provide usefulmeasurements-provided that the measurements are clearly recognized as approximations.

Determining map scale. Scale equates distance on the map or image with distance onthe ground. The scale of a map or image must be established before any features ordistances can be measured. Without that equivalence, anything measured on the mapor image relates only to that map or image and not to the world it represents. Theaccuracy of the scale determines, to a great extent, the accuracy of measurements made

81

82/ Chapter 12: Measuring Maps and Images

with that map or image. No matter how good a map or image is, if the scale isincorrect, measurements made on that map or image will also be incorrect.

Scale is determined by measuring the distance between two clearly visible features onthe map or image and finding the ratio between this distance and the real distance, onthe ground, between the same two features. (Measuring the distance between twopoints on a computer map is discussed in more detail on pages 84 through 86.) The realdistance can be found either by making a measurement in the field or by measuring iton another map or image for which the scale is known.

On this scanned map, the distance between twocros'roads k cA -m. The actual dise-cebetween the crossroads is 1.2 km. The scalefor this map is 2.75 cm per km. It can beexpressed as a ratio:

|3 cF m 3 1: 363,636

- 1or as a graphic:

0 < A 4 4. A O 1 2 3 4 5I I km

Whenever possible, this comparison and calculation should be done at least twice, usingdifferent sets of features. If the measurements give the same ratio or differ only slightly,it indicates that the scale obtained is reasonably reliable. (If the ratios differ slightly, theaverage should be used.) If the measurements give very different results, it means thereis a problem. In this case it is often best to choose different features and to redo thecalculations. Once the ratio has been calculated, it is generally represented on the mapby a graphic. All maps and images, to be truly useful, should include a graphic scale.

Determnining resolution. Resolution is a measurement, used only with raster maps andimages, that is closely related to scale. Resolution specifies the size of the square parcelof land represented by one pixel. For instance, a raster image may have a resolution of10 meters, meaning that each pixel in the image represents a square parcel of land 10meters on a side. Resolution is calculated much the same way scale is, with oneimportant difference. When calculating scale, the distance between two features ismeasured on the map or image and compared with the real distance. When calculatingresolution, the number of pixels between two features is counted and compared withreal distance. Making a pixel count is easiest when the line between the features iseither exactly horizontal or exactly vertical. If the line is at a 450 angle from either


horizontal or vertical, an A straight linear feature in rasterequivalent count can be made by representation is shown as acounting the pixels and sequence of white pixels. An y te s straight line has been drawnmultiplying by the square root of 2 between the centers of the(approximately 1.4142). The result U endpoints showing the true lengthof this multiplication is the of the feature. A second, brokennumber of pixels it would take, line is shown, connecting the centerarranged side by side, to cover that of each pixel. This second linedistance. In any other case, the emphasizes how the pixels zigzage n aback and forth in what is known asstaircase effect that is part of raster - the staircase effect.representation can cause an error.That is, in order to approximate aline at an angle, the pixels zigzagback and forth across it. Becauseof the zigzag, a simple countalways overestimates the numberof pixels required to represent anequivalent distance. Somesoftware will measure the distancebetween the first and last pixel in asegment and give an equivalentpixel count; knowing thesoftware's capabilities beforescanning is helpful.

One way to address the problem ofcomputing resolution is, before 42.80 5 52.60scanning, to measure the distancebetween two known features on A well-defined stretch of road whose length isthe paper map or irnage. A line of known has been traced in white. The softwaretexpapery mhap orgt imagte. Abie ofused has measured two things: the length of theexactly that length can then be segment in "pixel units" (the unit of measurementdrawn at the edge of the paper is the length of the side of one pixel) and the anglemap or image, either parallel to, or of the segment, measured from the vertical. Theseat right angles to, the direction of measurements can be used to calculate resolutionscanning. The scanned image will and orientation.then include a line with the proper CALCULATING RESOLUTION:orientation representing a known The actual length of the road segment marked indistance. The number of pixels in the white is 35 meters and the software used hasline can be counted and the resolution measured the length of this segment in the imagecomputed by dividing the distance by as equivalent to 42.8 times the side of one pixel.the number of cells. Resolution can be calculated:

RESOLUTION =

35/42.8 = 0.82 meters per pixel side


Determining orientation. Orientation CALCULATING ORIENTATION

relates the map or image to compassdirections. By convention, maps and A

images are generally oriented to the 52.60 565'

north. That is, a line pointing to thetop of the map points toward north.The orientation of a map or image can Angle measured Angle measured

be determined by comparing the on image on ground

orientation of a line drawn betweentwo features shown on the map and 390

the compass bearing of that line in thereal world. In some cases, an North, asshon JLn Northapproximate orientation is sufficient, shown

especially if the map is not going to be inage L>.used for something like navigation orcalculating solar angles. A majorconsideration in any case is that all themaps and images covering a given area This example refers to the image shown on the

be oriented the same way so that there previous page and is used to illustrate calculating

is consistency among sources. If a map resolution.

or image shows a major deviation inorientation, it would be worthwhile to Comparing te angle measured on the image with

the angle measured on the ground .shows arescan it. difference of 3.9°. For many applications, this

amount of deviation is acceptable and does notaffect the usefulness of the image.

Linear measurements: distance, length, perimeter. Linear measurements include: the

distance between two points; the length of a feature, such as a road or a pipeline; and

perimeter, which can be thought of as a path around an areal feature such as a body of

water.

Depending on whether the map is in raster or vector form (scanned and satellite images

are always in raster form), length measurements are made in different ways. In vector

format, the distance between two points can be obtained by simply drawing a straight

line between those two points; the software will automatically provide the length of that

line. The length of a feature that can be represented as a series of line segments is

calculated as the sum of the length of all the segments. Examples include rivers, roads,

and canals, or perimeters of areal features such as settled areas or plant communities.

In raster format, there are three ways in which linear measurements can be made.

Different software packages have different capabilities: some use only one method;

some use more. The simplest but (in most cases) least accurate method is to count

pixels and multiply the count by the resolution (the length of the side of a pixel).(Text continues on page 86)


MEASURING DISTANCE IN A RASTER MAP/IMAGEIn the examples that follow, three different methods are used to calculate the length of the featurerepresented as a sequence of white pixels. In each case the resolution of the image is 0.82 metersper pixel side. Normally, someone using software to measure features in a raster map or imagewould not even be aware of this type of computation. These examples are included only toillustrate the different methods used and the different answers they can give.

METHOD 1:There are 15 pixels in the white segment. Measuringdistance by simply counting pixels would give a length A Oof 15 pixel sides in the image. This gives a length on theground of:

15 x 0.82 = 12.3 meters

METHOD 2:Going from center to center of the pixels making up thewhite segment gives a count of 10 diagonals and 4 timeshorizontals. Each diagonal is equivalent to 1.4142 timesthe length of a side of a pixel. This method gives alength of the segment in the image as [(10 x 1.4142) +4] /= 18.1 times the length of a pixel. This gives a length onthe ground of:

18.1 x 0.82 = 14.8 meters

METHOD 3:Measuring the distance between the center points of thefirst and last pixel without paying attention to theintermediate pixels is like measuring the length of thediagonal of a triangle. In this case, the horizontal side ofthe triangle is 14 pixels long and the vertical side is 10 E

pixels long. The formula for the length of the diagonal -Sgives: ____._--

2. 2Length= ' 14 + 10O

=4196 + 100

= 296

= 17.2 pixel sides

This gives a length on the ground of:

17.2 x 0.82 = 14.1 meters


Distances measured this way are accurate only when the segment being measured isexactly horizontal or vertical. A second and somewhat more sophisticated methodinvolves measuring the distance between the centers of the pixels making up the linearfeature. Because all the pixels are arranged in a grid, the distance between the centersof two pixels is always equal either to the length of the side of one pixel (when thepixels are beside each other or one on top of the other) or to the square root of two(approximately 1.4142) times the length of the side of one pixel (when the two pixels areon a diagonal.) This method is more accurate than simply counting pixels, but still haserror because of the staircase effect. The third and most accurate method is to measurethe distance between the two endpoints of a linear feature in the same way vectorsoftware does (the software ignores all the intervening pixels and just looks at thestraight line formed by connecting the centers of the two end pixels).

Raster software packages vary both in how they measure length and in how automaticthey are. Some packages will measure the lengths of the individual segments makingup a linear feature and sum them; others require that the user take each measurementand then add the measurements up.

Area measurement. In raster maps and images, areal features are shown as groups ofpixels or cells. When resolution is known, the area of such features can be computed bycounting the pixels and multiplying the pixel count times the area represented by eachpixel. For example, if an image had a resolution of 3 meters-that is, each pixelrepresented a parcel of land 3 meters on a side-then the area represented by each pixelwould be 9 square meters. If a feature were made up of 1,652 pixels, then its areawould be 1,652 x 9, or 14,868 square meters. Software programs vary in how theyhandle partial cells.

- ~ ~ ~ ~ ~ ~~ U I I-sIIIII1.gIIl-- -L - L. I .. IL II )ot-

In a raster representation, an areal feature is shown as a group of In vector representation, an arealpixels. The area of the feature is calculated, in the simplest case, by feature is shown as a closed polygon.counting the pixels and multiplying that count by the area To calculate the area, the polygon isrepresented by each pixel. divided into square cells.

In vector maps, an areal feature is represented by a closed polygon. To compute thearea of the polygon, the software divides it into small square cells, counts the cells, andapplies a conversion factor determined from the scale. just as with raster maps,


software programs vary in how they handle partial cells. In some cases, any partial cellis included in the calculation as a half cell. Other programs are much moresophisticated in how they calculate partial cells and therefore give more accurateresults.

Measuring buffers and corridors. Buffers and corridors are measurements that areusually generated automatically by software and show the nearest distances betweenmany points. Buffers show regions at specified distances from a point or areal feature.For instance, software could be used to generate buffers showing regions less than 500meters, between 500 and 1,000 meters, and more than 1,000 meters from existing orplanned standpipe locations for providing drinkable water. The buffers could then becompared with other information such as the location of dwellings.

Corridors show regions at specified distances from a linear feature. For instance,corridors could be generated showing regions 1, 2, 3, 4, and 5 kilometers from a majorroad. Again, these measurements could be combined with other information such ashabitation and land use.

1~T&

~OW

--- -- ---

rein of spcfe ditne on eihrsd ftiod

LR'S *} * I

*,x I -1 11,; [ 2H

On the left is a scaned image of an air photo of an urban area. A major road through this area hasbeen traced in white for visibility. On the right is a corridor, generated by software, showingregions of specified distances on either side of this road.

Measuring is one of the most common and important map uses. Althoughmeasurements can certainly be made using paper maps and images, the same types ofmeasurements can often be made more easily, more rapidly, and more accurately withdigital maps and images.

Part 3: OUTPUTOnce processing is complete, the next step is creating output. In its most general sense,output is the result of using the computer to acquire and process information. Twotypes of output are discussed here: printed output and screen output.

Printed output (sometimes called hard copy output) is paper output and includes mapsand images, either printed for independent use or integrated as part of otherdocuments. Some basic considerations such as selecting subscenes and addingannotation are discussed in chapter 13, and a technique is presented in chapter 14 forcreating paper mosaics when the printed map or image is larger than standard printerpaper.

Simple Computer Imaging and Mapping deals only with hard copy output from laserprinters. Other devices, such as pen plotters and electrostatic and ink-jet printers can beused for hard copy output. However, none of these other devices meet all the criteria ofwide availability, low cost, and ability to be used for nonspatial output, such as text. Aswith input and processing, the intent is not to present an overview of the availabletechniques and equipment, but rather to give a limited selection of output techniquesthat are quick, cheap, and easy.

Screen output is the image or images seen on the screen. At every step of scanning,processing, and printing, screen images are used to choose and manipulate theinformation. Viewing is generally a capability built into the software used to scan,process, or print. However, screen output can also be an end in itself. Screen output isparticularly useful in presentations and demonstrations as a means of presenting visualinformation. Some of the capabilities of screen output are discussed and some things tolook for in the software used for screen output are pointed out in chapter 16.

A form of hard copy output: digital mosaics printedon multiple sheets of standard printer paper, then

L -assembled into paper mosaics and laminated.

89

Chapter 13Hard Copy Output-Printing Maps and Images

. ^. 5What is hard copy? Hard copy is the term used for printedcomputer output. Paper copies of maps or images-hard

A. copies-are often the most important result of processing~ 't e computer maps or images. The great flexibility and range of

printing options available with computer maps and images is a[7 large part of their power and attractiveness.

J Why is hard copy output important? The greatest advantage- - printed maps and images have over their digital counterparts is

that they are permanent and exist independently of computeruH hardware, software, and power sources. They are familiar and

easy to transport, mark, and produce in multiple copies.

Printed maps and images and digital maps and images fulfill different needs. Theprinted form is generally most convenient as a communication tool and for use in thefield. The digital form has many advantages for updating, enhancing, and analyzing-and for producing paper copies.

What should be included in a paper map or image? Printing a map or image usuallyinvolves more than just printing a picture of what is seen on the screen. In many cases,a subscene, rather than the entire map or image, must be selected for printing. The scaleof the printed output also needs to be selected, and annotation should be added.

Some software can print just a selected portion of a map or image, rather than the wholefile. This ability to print a selected subscene is often useful. Image files in particular canbe very large. Printing only the area of interest saves both paper and time. Whenselecting a subscene for printing, try to include at least one distinctive feature, such asan easily identifiable bridge, intersection, coastal feature, or river bend to make it easierto locate the subscene within the general area.

Most software used for maps and images offers a choice of scales when printing. Scaledefines the relation between distance as shown on the map or image and distancemeasured on the ground. Things to consider when choosing a print scale include:What is the quality and resolution of the digital file? How will the printed map beused? How much detail should be visible? How big will the printed map be? Is itdesirable to match the scale of other maps or images for the area?

91

92/ Chapter 13: Hard Copy Output-Printing Maps and Images

Selecting a subscene (outlined inwhite) from a digital mosaic of or

air photos. Only the subscenewill be printed. .

l ........A . . .. ......... ......

Sometimes the best solution is to make more than one printout, at different scales. For

example, subscenes printed at a relatively large scale are often paired with printouts of

a much larger area, printed at a smaller scale. The subscene shows detail, while the

printout of the larger area helps to locate that subscene.

Software packages vary considerably in how annotation is added and in how much

flexibility is given in doing so. However, there are certain types of annotation that all

printed maps and images should have. These include a title, a north arrow, a scale, and

(for maps) a legend. The title names the area shown. It may also give informationabout the type of map or image-for instance, "Population Density" or "Satellite Image,

August 1989." The north arrow shows the compass orientation of the map or image.

T'he scale relates distances shown on the map or image to real distances. Scale can be

expressed either as a ratio or with a graphic scale bar. The ratio 1:200,000 would meanthat 1 centimeter on the map represents 200,000 centimeters-or 2 kilometers-on theground. A graphic can be used to show the same information. Printed maps andimages are often reduced or enlarged, so a graphic scale, which is reduced or enlarged

along with the map or image, is almost always more useful than a ratio, which is true

only for the original size. Maps (as opposed to images) should include a legend, whichlists and defines the symbols used in the map. Other types of annotation that can be

useful include labels for both natural and man-made features, and a reference grid.


Catumbela Neighborhoods

'-f2 ,,- -/r -

fi II II II H . < --HI

AM'

- - - 2 11-saof f

r -.

Kioetr 128,346

Ths irpht/ heatcmaiclds bscantain a- titl, a: legnd a ot

r ,a Planned Settlement N

Spontaneous Settlement 'OI Kilometers 1.28,346

This air photo/thematic map includes basic annotation: a title, a legend, a northarrow, and a scale, shown in both graphical and ratio form.

94/ Chapter 13: Hard Copy Output-Printing Maps and Images

What are some other printing considerations? Depending on the printer being usedand the resources available, there may be a choice of paper quality, color, and size.Most printing is done with black ink on white, letter-sized, photocopier-type paper.However, printers are increasingly able to handle both heavier and larger papers. Also,it is now possible to get colored toner cartridges for laser printers, so that printing cannow be done in colors other than black, such as brown or blue. Many printers can printon acetate transparencies as well as on paper. Maps or images printed on thesetransparencies can be used with overhead projectors. Printed transparencies are alsouseful when digital maps have been created in thematic layers. Layers can be printedout separately and combined in different ways to illustrate different spatial objects andrelationships.

Sometimes printed maps or images can be 7 .

made more useful by laminating them- IIrenclosing them in a thin, transparent plastic -

film. The best lamination is done with aspecialized machine, but serviceable -lamination can be done by hand using -adhesive-backed transparent plastic. - -

A laminated map or image is more durableand is therefore particularly useful forfieldwork. A laminated map or image canalso be used and reused for marking, -- |

especially when erasable markers are used.Once the information has been transferred, 4the markings can be erased and the map or . 3image used for something else. However, -lamination does have some drawbacks. -First, the plastic surface is shiny and subject i -:to glare, and in some lighting conditions a , < -

laminated map or image can be difficult to I = -read. Second, the plastic surface is slippery, -- -

and laminated maps have a tendency to slip Laminating an air photo mosaic using specialized

off surfaces easily. Third, the plastic is equipment.

stiffer than paper so that a laminated mapor image cannot be folded and must be rolled. This sometimes makes transportingthese maps and images difficult, especially when space is at a premium. Finally,lamination takes both time and money. When either resource is in short supply,lamination will not be as attractive an option.


Hard copy output is probably the single most important product of digital maps andimages. A digital spatial database can serve as a source of inexpensive printed mapsand images. The tremendous flexibility of the digital form makes it possible tocustomize each printout by selecting the subscene, scale, and annotation. Thisdiscussion has dealt with some of the general issues involved in printing maps andimages; chapters 14 and 15 present two more specific cases, creating printed mosaicsand including maps and images in reports and other documents.

Chapter 14Technique-Creating a Printed Mosaic

When a map or image is printed, the combination of thescale and the subscene that have been chosen often results in

; i,a output that is larger than standard printer paper. Such a- map or image can be printed in sections and the sections

assembled and glued to form an apparently seamless whole.This technique is very useful when printing a digital mosaic

4 or a single large file, such as an air photo scanned at veryhigh resolution.

A printed mosaic can also be created from a collection ofindividual scan files that cover a given area but have notbeen joined in a digital mosaic. Each file is printedindividually and then the printouts are joined together.

The HardwareA personal computer that has adequate RAM and a monitorcapable of displaying at least 16 levels of true gray.

Personal computerA removable mass storage device that is capable of storingthe large files typical of large maps and images.

Removablemass storage device

LI __vz A laser printer that supports gray tone printing.

Laser printer

The Softwarel M System software. This is the basic software that runs the

computer and allows other software packages to be used.

System softwareSye sImage processing software for printing images and raster

maps. The package should allow control over the scale andthe subscene selected for printing.

Image Processingsoftware

-< Drawing software to print maps in vector format. If thesoftware has graytone capabilities, it can also be used forimages.

Drawing software

97

98/ Chapter 14: Technique-Creating a Printed Mosaic

Creating a Printed Mosaic

A Import the file orfiles to the softwareused for printing. .......

2 Select subscene and scale.t J ~Print individual pages. = $

Arrange pages.

4 Trin and glue.


There are four steps in creating a printed mosaic. First, import the individual scan files,image, or map to the software used for printing. Second, choose scale and subsceneand print the image or map. Third, assemble the printed sheets in their proper order.Finally, trim and glue together the printed sheets to create the paper mosaic.

If the image or map has beenl processed (for example, enhanced,

tthe file or assembled in a digital mosaic, or1 to the software traced) then it will probably alreadyused for printing . . be in the file format necessary for

printing. If, however, the files beingprinted are individual scan files thathave not undergone any processing,

then they will have to be imported to the printing program. To do so, open the scannedimage from the program used for printing. The image can then be displayed andprinted and finally saved in the format employed by this software.

Once the map or image is opened inSelect subscene and scale. the appropriate software, select

-------- print parameters. These can includen ichoosing a subscene for printing

and the scale of the printed map orimage. Different programs vary

considerably in the amount of control they give the user over printing format. Forexample, it may be possible, at the time of printing, to add and position annotation or todetermine the position of the printed image on the pages. Exactly which parameterscan be set by the user and the range of options available will depend on the software.

A subscene selected from a digital mosaic for printing.


j - 4 Once printing is complete,arrange the individual pages

- ;on a flat surface in the proper

3 Arrange pages. _ order. When a single largeE map or image file has been

IN 7 - printed, the individual pagesi. - are generally printed in a

logical order and with a-- --- standard white border. When

the pages are placed side byside, the map or image willline up just as it did when

.k a .displayed on the screen.

j -When the software permits,- 'I print with some overlap

between the sheets. Thiseliminates the problem ofwhite join lines in the finishedmosaic.

The task is somewhat moreIndividual difficult when printing andscan files L '5 .m assembling individual scan

arranged and fo - ' . files because there is noready for predetermined printing order.trimming and -

gluing. Also, the amount of overlap- - - rwill probably vary from page

to page. Assembling andpositioning the pages requirespatience and a great deal ofclose attention.

- _ -- Once the printed pages have__ A been assembled, trim them

-- and glue them together.1~ Trimming removes white

rborders and excess overlap so

4 2 Trim and glue. - that the pages can be alignedproperly and with a

w - . .minimum of bulk. Trimmingse _ can be done with hand-held


scissors, but a blade-type paper cutter is strongly recommended, both for creatingsmooth, straight cuts and for speeding up the process. If this type of paper cutter is notavailable, a razor-type blade tool aligned against a steel ruler can be used with goodresults.

A trimmning and gluing schematic.The shaded areas are those to be cut I _ [ I

off. The diagonal cuts at corners are - X

to minimize the number of layersand prevent lumps in the finishedmosaic.

When printed from a single file, the borders and the alignment of the individual pagesare quite standard. This makes it possible to use a trimming and gluing scheme like theone in the above diagram. The diagonal cuts at the corners are used to reduce thenumber of layers and prevent bumps in the finished mosaic.

When assembling pages printed from individual scanfiles, it is not as easy to use a standardized scheme,although the same considerations apply. There shouldbe about 2 to 3 centimeters of overlap between thepages, and care should be taken to minimize thenumber of layers so that the finished product is as -

smooth as possible.

In either case, the pages should be glued together usingpaper glue or paste. A commercially available gluestick works well. Because the glue is relatively solid, it .-is easy to apply, doesn't spread, and doesn't soakthrough the paper. Rubber-type cements are also easyto work with-the cement can be peeled off without -

damaging the paper if it is applied somewhere it is not Tools for creating a printed mosaic:wanted. A small roller is a useful tool during gluing. a glue stick and roller.


Once two pages have beentrimmed, positioned, and glued,running the roller up and downthe glued seam flattens the seamby eliminating any air bubblesor glue lumps and assures thatthe pages are firmly joinedtogether.

k -

AL! The finished mosaic can be usedas is" or it can be laminated to

increase its durability. WhenA . . commercial photocopying with

large paper is available, the--- -mosaic can be photocopied to

Gluing together two pieces of a printed mosaic. obtain multiple, seamlesscopies. (This type ofphotocopying can, however, be

r -expensive.) These photocopiescan also be laminated, provided

- they do not exceed the sizelimits of the laminatinglequipment that is used. Forboth the original mosaic and

"Oa ,-any photocopies made of it,appearance and durability willbe improved if relatively high-quality paper stock is used.

Rolling a seam after gluing. The glue stick and trimmedportions of the mosaic are in the foreground.

Chapter 15Including Maps and Images inReports and Other Documents

o How are maps and images included in documents?Maps and images are intended to communicateinformation and frequently are important additions toreports and other documents. There are two commonways in which maps and images appear in documents: as

. supplements or as illustrations that are integrated into thedocument.

Supplements are generally larger than the rest of the document and are folded andplaced in the back of the document. The size of the supplement is chosen to show thearea and the level of detail considered best for communicating spatial information.However, including supplements can significantly increase the cost of producing adocument. Supplements are also inconvenient because they are separate from the bodyof the document and require a relatively large flat area on which to unfold and viewthem. In addition they often get separated from the document and are lost.

When maps and images are integrated with text, they must meet one very importantcondition: they must fit on a standard page, or, at most, on two facing pages. Mostreports are printed on what is known as letter sized paper. In North America, thismeans paper that is 8 1/2 by 11 inches; in Europe, the standard is 21 by 30 centimeters.Once margins have been taken into account, the maximum space available for a map orimage is about 6 1/2 by 9 inches or 16 by 25 centimeters for a single page, or 13 by 18inches or 32 by 50 centimeters for a double page-considerably smaller than most mapsand images. Drawing or image processing software can be used to condense the map orimage to the proper size, and this condensed image is then imported to whateversoftware is used to produce the document.

This chapter discusses the second method of including maps and images in documents:condensing them and integrating them with the rest of the document. These techniquescan also be used with graphics other than maps and images-for example, thephotographs that have been used as illustrations throughout this publication. Thesephotographs were scanned, enhanced, and then resized before being integrated into thetext.

How are maps and images condensed? There are two main ways in which maps andimages are reduced to a usable size. The first way is to select and include only thespecific area discussed in the report. (This process is sometimes referred to as cropping.)

103

104/ Chapter 15: Including Maps and Images in Reports and Other Documents

The second way to control the size of the map or image is by choosing an appropriate

scale. The smaller the scale, the smaller the map or image. The two methods are often

used together.

Both techniques have costs. When only a portion of a map or image is shown, the areal

context can be lost. For instance, if a map or image is cropped to show only theneighborhood under discussion, it may be impossible to tell where in the city the

neighborhood is located or where it is in relation to features such as bodies of water,

cultivated areas, or major transportation routes. Contextual information is often useful

and should be included in some way.

The cost associated with choosing a small scale is loss of detail. As the scale becomes

smaller and smaller, it becomes more and more difficult to identify individual features.The processes involved in publishing most reports make this problem even greater.

Typically, reports are printed using word processing or desktop publishing software

and then are photocopied, rather than being produced with higher quality, higher costtechnology such as typesetting.

-J ,Clarity and detail can be lost inr~

both printing and photocopying. Iftoo much degradation takes place,

r -the final image will fit on a page,but it will not show what it wasmeant to show.

A subscene from an air photo hasbeen copied at four different scales.As the scale and size of the image

l -gets smaller, it becomes more and

- -more difficult to identify features.

75%

50%

25


How can these costs be offset? The costs associated with making page-sized maps andimages can, to some extent, be offset by using paired images. When the areal context ofa map or image has been lost, it can be paired with a locator map. A locator maptypically shows a larger area, with very little detail. It does, however, show majorfeatures, such as coastlines, rivers, or political boundaries, that are needed to locate themap or image in question. This technique is commonly used in newspapers andmagazines. The two maps or images can be placed on the same page or on facingpages, depending on the size and shape of the particular maps and the format of thepublication.

Using a small locator map/image to provide arealcontext for an image. The larger image shows the city of

.s -L Lobito; the small inset shows the Lobito-BenguelaX coastal corridor and surrounding area. Positioning the

locator image over the ocean on the larger image savedspace without losing urban information.

I,.rrr

44

. ,

w.'~~4"- 4'i'. a/

V- a R' ' 5 a ;

p. r I

f' .r -r0'> f :l --

af .. 1 t~ 4 4

- p *1 ' - .-

ffi F F t; 'I * -' ^

106/ Chapter 15: Including Maps and Images in Reports and Other Documents

Pairing an image with a smaller one to show detail. The larger image shows the Lobito-Benguelacoastal corridor and surrounding area, while the inset shows the detail in a scanned air photo of thearea. The inset is positioned over the ocean to save space without losing regional information.

-- v - -

. ;. k(.

v - .A

A- t'


When a map or image does not show enough detail, it can be paired with one or moresubscenes shown at larger scale. These subscenes are called detail maps or zoom maps.The larger scale of the detail maps compensates, to some extent, for the combinednegative effect of smaller scale, and the loss of clarity through printing andreproduction on the original map or image. Identifying features in the subscenes,where they are shown more clearly, can help to identify these same features in theoriginal. As with the locator maps, these detail maps can be placed on the same page asthe original map or image, or on a facing page.

When using paired maps or images, the idea is to balance two things: areal context anddetail. When the primary map or image is large-scale and shows detail, the locator mapprovides the context. When the primary map is small-scale and shows adequate arealcontext, a detail map can help make up for missing detail. In both cases, the basic ideais the same; only the emphasis changes.

When detail is lost, another technique that is sometimes useful is rotating a map orimage 900. Maps and images are often wider than they are tall. By contrast, documentsare almost always bound so that the pages are taller than they are wide. Rotating themap or image 900 makes better use of the space available and a somewhat larger scalecan be used. The larger scale results in a clearer map or image.

.4,'

N

i,, tR ' > v, ,

This subscene is wider than it is tall. This is one way of including it in a report.

a a .Ca'. 2' __ _ _ __ _ _ _ __ _ _ _ a

fl* r t 4''"' w '*

S £. , _

r r-A

S0

I, ., , '' S C 'a;,J-'jr%

-r i ot a t a 'oarger' ca e.

Thesuscneshwnonth pevou pgehasben otte 9°.Inths rinttin i cn e islaedata arerscle


Are there other uses for a one-page map or image? A map or image condensed to aone-page summary can find uses outside of documents, for example, serving as areference when a printed mosaic is assembled. Having the entire region shown on asingle page, even with very little detail, is very helpful when putting the printed pagesin the proper order. A one-page summary can also function as an index for a collectionof maps or images. The summary can show not only the entire region covered by thecollection, but the area shown by each individual map or image. In both cases, the one-page summary complements the other maps or images, showing the general picturewhile the mosaic or the collection shows the detail.

The key to creating and using one-page maps and images, whether in documents or forother purposes, is to remember the primary purpose of any map or image: to presentand communicate spatial data. The techniques used to create one-page maps or imageshave drawbacks, which can, to some extent, be compensated for. When a reasonablecompromise can be reached between achieving the desired size and showing thenecessary information, the one-page summary is a valuable tool. If, however, thatcompromise proves impossible, it is better to omit an illustration that at best maycontribute very little, and at worst may prove confusing.

Chapter 16Screen Output-Viewing Digital Maps and Images

What is screen output? The text and images seen on a computerscreen are actually a form of output, just as printed text andimages are. A computer processes everything digitally-asnumbers. That digital information is then translated into someform of output that humans can interpret and use. Both screen

I 3 and printed output are important, and they can, to a great extent,., complement each other. Printed output was discussed in chapters

'-=___-==--= 14 and 15; this chapter discusses various aspects of screen output.

Viewing maps and images on the computer screen is an importantIIJIHlllllWIIIUUlhfllIIllhiUUDI part of scanning, processing, and printing them. However,

viewing a screen image can also be an end in itself, just as viewinga paper map or image is. This chapter deals with the second type of viewing.

Maps and images can be viewed using many different types of software. There aresome relatively new packages, known as viewing software, that have been createdspecifically for viewing graphics, including maps and images. Many other types ofsoftware also have viewing capabilities. Scanning, drawing, and image processingsoftware all allow some type of viewing. Hypertext-software used to organize, search,and present information-can also be a very effective viewing tool.

Why is screen output important? Viewing is the single most common and importantuse of maps and images, whether in digital format or on paper. Viewing maps andimages on the screen is essential for creating, processing, and printing them. Viewingthe finished product on the screen is also important for interpretation and presentation.

In certain respects, viewing screen output has advantages over viewing paper output.Screen resolution is often much better than that of available printers. This means thatthe screen image is much clearer and shows detail better than the correspondingprinted map or image, making it easier to identify features.

A screen image is also seen by the human eye somewhat differently than a paper imageis. The computer screen emits light, whereas paper reflects light. The backlit, luminousquality of a screen image can make something like an air photo seem more three-dimensional, more "real."

111

112/ Chapter 16: Screen Output-Viewing Digital Maps and Images

Although we have dealt almost exclusively with black-and-white or graytone maps and

images, color is often an important element. High quality color printers for personal

computers are available, but they are very expensive. Color monitors, however, are

much more affordable. When digital color data such as multispectral satellite imagery

is available, screen viewing may be the best and most economical way to see it.

Screen viewing also has a dynamic component that is missing when viewing paper

output. The viewer can both pan (move across) and zoom (change the viewing scale of)

a screen image. Screen viewing makes it very easy to focus on a specific area of a map

or image and to view the same map or image at different scales. Both panning and

zooming are dynamic processes that can provide a sensation of motion. Many people

find these capabilities both useful and stimulating. It is hard to put a value on this

stimulation factor, but it is common to find that it contributes to more effective map use.

People see more in the map or image and see it more quickly when they enjoy viewing

it and are actively involved in the process.

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Naturally, screen viewing has disadvantages as well as advantages. Screen viewingrequires a computer. Computers are expensive (at least compared with the cost ofprinted maps and images) and require a special environment, one that is dry, dust-free,with constant temperatures, and a reliable power supply. This means that screenviewing is not appropriate when funds are limited or in many field situations. Screenviewing also requires being comfortable with computers and the software used forviewing. If these requirements aren't met, screen viewing is frustrating and of limitedvalue.

What capabilities are important for screen viewing? Screen viewing can be useful andenjoyable, or it can be tedious and relatively useless, depending on the capabilities ofthe viewing software. For example, scanning software and drawing software can bothbe used to look at a map or image, but, because they are intended for very differentfunctions, the ways in which that map or image can be viewed will be quite different.Differences can include the amount of the screen used to display the image, the speedwith which an image can be manipulated, and the scales available for viewing. It isuseful to keep in mind the capabilities necessary for successful screen viewing whenselecting viewing software.

One of the most important capabilities is speed of display. Graphic data, because of itsnature, requires large amounts of storage space and computer memory. This meansthat when a graphics file is opened, it can take quite a while to display on the screen.Periods of time that in other contexts seem quite short-15 or 30 seconds-becomeunacceptably long when spent staring at a computer screen waiting for an image todisplay. For viewing purposes, it makes sense to use a package that displays graphicfiles quickly, even if that means that other abilities, for instance, the ability to enhanceor modify images, are missing.

Also important is the ability to move around within a map or image and to zoom in andout. Quick, easy pan and zoom are essential to effective screen viewing. Perspectiveviewing and animation of the landscape based on a constantly moving viewing positionare increasingly available features. These capabilities become particularly importantwhen viewing very large graphic files using regular-sized monitors. Pan and zoom areminimum requirements, while perspective viewing and animation are useful, but not ascritical.

The ability to display more than one image or map on the screen at a time is also useful.For example, different representations of the same area can be compared, and some ofthe techniques discussed for printed maps and images, such as using a locator map or adetail map, can be used on the screen.

When working with a spatial database-a group of related maps and images-theability to organize the files in a meaningful way is important. This organization allows

114/ Chapter 16: Screen Output-Viewing Digital Maps and Images

the user to move between related files, finding and displaying files quickly and easily.

Hypertext and visual database packages are two types of viewing software with

browsing capabilities.

It is very difficult to make specific recommendations about obtaining and viewing

screen output. A great deal depends on the type, size, and number of maps and images

that are being used, the software on hand, the funds (if any) available to acquire new

viewing software, and the motivation of the users. Perhaps the best that can be done is

to point out the importance of screen output and to highlight the capabilities that can

make it most worthwhile.

Lobito

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An example of viewing more than one file at the same time. A line map and two images at different

scales, all of the same area, are shown.

AfterwordAn Example That Combines TechniquesUp to this point, techniques have been presented individually. However, mostapplications require using more than one technique. This afterword shows, in graphicalform, the combination of techniques the authors have used most often.

Scan the air photos. Save the scanned image files on a mass storage device.1 Create a printed index listing the files. In this example, each file name givesthe city, photo number, and file format.

Air photos

Scanner Computer Mass Storage System ScanningDevice software software

SCANNED AIRPHOTOS

LOBITO-236-TIFF

LOBITO-241-TIFFLOBITO-242-TIFF Digital imagesLOBITO-245-TIFFLOBITO-246-TIFFLOBITO-247-TIFF

115

116/ Afterword: An Example That Combines Techniques

Enhance the scanned images one by one. The scanned images are importedto image processing software for this step. In this example, one goal ofenhancement is to bring out information in settled areas. No attempt ismade to match the brightness of adjacent images.

Scanned image

Computer Mass Storage System Image ProcessingDevice software software

Enhanced image

r.... ............... ..... .. ... ... .


Create a digital mosaic using image processing software. The resulting file3 is too large to be convenient for most processing and output and isintended mainly as a reference and source of subscenes.

- - ~ 5 Individual scannedimages

-- D

Computer Mass Storage System Image ProcessingDevice software software

-- Seamless digitalmosaic

............... ........... ..........

118/ Afterword: An Example That Combines Techniques

Select a subscene, print the individual pages, create a paper mosaic. Theprinted image map can be copied and/or laminated. Different subscenes,different scales, and different annotation can also be chosen.

Selected subscene

iL;l -=

Laser Printer Computer Mass Storage System Image ProcessingDevice software software

A Paper mosaic

City of Lobito Fkm.

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Appendix AGlossaryannotation. Text and symbols added to maps and images to label or explain features orto add useful information.

array. An orderly arrangement of objects or numbers.

backup. An extra copy, usually of a digital file, that is (ideally) stored separately fromthe original. The object of creating a backup is to ensure that if the original file isdamaged or lost, the information contained in it is still available.

brightness. A measure of the lightness or darkness of an image.

buffers. Regions at specified distances from point, line, or areal features shown in amap or image.

contrast. A measure of how different graytones are distributed in an image. In a low-contrast image, all the graytones are clustered in a relatively narrow range. In a high-contrast image, the graytones are spread out over a much wider range.

contrast stretching. An image enhancement technique that modifies an image bydistributing graytones across the entire available range.

coordinates. A set of numbers that defines the location of a point with reference to agrid or set of axes. Points represented in maps and images (which are two-dimensional)can be located by coordinate pairs.

corridors. Regions at specified distances from linear features shown in a map or image.

cropping. The technique of creating a subscene by cutting off portions of a map orimage.

delineation index. A pictorial index showing the relative position of air photos ormaps covering a given area.

detail map. A large scale subscene that is paired with a smaller scale map or image toshow detail. (See zoom map.)

device. The mechanical unit, such as the disk or tape unit, that a digital file is saved on.

digital. Representing information numerically. Often indicates information incomputer format. Digital files may be-and generally are-displayed visually forhuman use and interpretation.

digital image: An image represented by an array of numbers that can be read by acomputer. Digital images can be displayed visually for human use.

119

120/ Appendix A: Glossary

digital map. A map represented by an array of numbers that can be read by a

computer. Digital maps can be displayed visually for human use.

digital mosaic. A computer map or image formed by joining together two or more

smaller files.

display scale. The scale of a map or image displayed on a computer screen. (See scale.)

dpi. An abbreviation for dots per inch. Used to measure image resolution. (Seeresolution.)

enhancement. An image processing technique involving modifying the appearance of

an image in order to make it easier to interpret.

file format. The way in which digital information is arranged within a file. File formats

are given names (such as PICT or TEZFE). Different software programs vary in terms of

which file formats they can read and write.

file name. The name assigned to a digital file. A file name is needed to locate, display,

and save a digital file.

grayscale. An image type or a scanning mode that uses or identifies a range of grays.

ground confirmation. Observations made in the field to identify, prove, or disprove

feature classifications made using air photos or satellite images.

hard copy. Printed computer output.

histogram. A specialized bar chart used to show frequency of occurrence.

hypermedia. Software and electronic devices used to organize, search, and present

information in textual, graphical, and audio form.

image. A picture-for example, of the earth's surface. In this publication image can

include both air photos and satellite images.

image enhancement. (See enhancement.)

import. To bring a file created in one software package into another. In some cases,

this may involve loss of information.

input. Information in a form that a computer can use.

lamination. The process of encasing a printed map or image in clear plastic to increaseits durability and utility.

legend. Text added to a map, usually in the form of a table, listing and defining the

symbols used.

line art. An image type or scanning mode in which everything is shown as either pure

black or pure white.


locator map. A small-scale area map or image paired with a larger scale subscene, usedto locate that subscene.

manual mosaic. A printed map or image formed by joining together two or moreprinted sheets manually.

map. A drawing showing the location and characteristics of natural and/or man-madefeatures.

marking. Adding information by drawing symbols and text onto an existing paper ordigital map or image.

mosaic. A pattern or whole formed by joining together smaller pieces.noise. The part of a signal or image that does not represent information but is causedby limitations of equipment, processing, or recording.

north arrow. An arrow added to a map or image that points toward North asrepresented in that map or image.

orientation. The relationship between a map or image and compass direction.Orientation is generally indicated by a north arrow.

output. Information produced by a computer.

parameter. Characteristics of a process that determine how it will proceed. Forexample, scanning parameters can include scanning mode, resolution, scale, threshold,brightness, contrast, and scanning area.

PICT. A file format widely used for graphical data.

pixel. A discrete picture element.

polygon. An areal shape bounded by straight sides.

raster. A data format used to represent spatial information. The area in question isdivided into an array of square cells, and each cell is assigned a value representing thedominant characteristic of that cell.

raster map. A computer map in raster format.

reference grid. A system of vertical and horizontal lines superimposed on a map orimage to help locate and compare features.

resolution. The fineness or coarseness of the grid used to represent an image in rasterformat.

scale. The relationship between distance on the ground and distance as represented ina map or image. Scale is often represented either as a ratio or by a graphic scale.scanning mode. How a scanner interprets information. For example, scanned materialmay be interpreted as a range of grays or as patterns of pure white and pure black.

122/ Appendix A: Glossary

subscene. A portion of an image.

threshold. A parameter used during scanning in line art mode that determines when apoint is classified white and when it is classified black.

TIFF. A file format widely used for image data.

vector map. A computer map in vector format. Each feature is represented either as apoint, line, or polygon and each feature can be identified and manipulated as an object.

zoom map. A large-scale subscene that is paired with a smaller scale map or image toshow detail. (See detail map.)

Appendix BTaking Care of a Digital DatabaseWhat is a digital database? A digital database is a collection of information incomputer format. The term database generally implies some sort of organization orstructure to that collection. This publication presents some techniques for creating andusing a digital database, specifically a collection of digital spatial data. If the database isto be truly useful, however, there must also be techniques for organizing, accessing, andmaintaining it.

What type of care does a digital database need? It may help to think first of a morecommon database a library. A conventional library is (primarily) a collection ofprinted materials, such as books, magazines, and pamphlets. A library would still be alibrary-although a fairly useless one-even if all the publications had blank covers andwere placed randomly on the shelves. A library is a useful database when: everypublication has an identifying name; all the publications are organized according to awell-defined system; library procedures try to ensure that publications can be usedwithout being lost; old materials are constantly being removed and new materialsadded; and there are indexes showing where different publications can be found. Theseare the same things a digital spatial database needs. Specifically, files need identifyingnames; they should be organized in a functional way; backups should be created so thatthe files can be used without risking losing that information; the database needs to beperiodically updated and old files removed; and indexes should be created to help findinformation as it is needed.

Choosing names for digital files. Just as every book has a title, every digital file has aname. Increasingly, computer systems give users great flexibility in naming files,allowing names of more than one word, made up of a variety of characters. Thechallenge is to choose file names that are functional. One goal to keep in mind is thatthe file name should give a person who has not created that file a reasonably good ideaof what the file contains.

Most of the files discussed in this publication are digital versions of maps, air photos, orsatellite images. Their file names could include the place name of the area depicted, thesource of the data, the date the data was acquired (if this is relevant) and the file format.A file created by scanning a land use map of Lobito might be named "Lobito Land UseMap-PICT," where PICT is the file format used to store it. A file created by scanning anair photo taken over the same city might be named "Lobito-8/87-233-TIFF," where"8/87" is the date the photo was taken, "233" is the serial number of the particularphoto, and "TIFF" is the format of the file.

123

124/ Appendix B: Taking Care of a Digital Database

When a digital database is first set up, an initial period of experimenting with filenames is almost inevitable. Thinking about file names before the fact will notnecessarily eliminate the experimental stage, but it is almost certain to make it moreproductive.

Organizing digital files. There are two basic ways to organize digital files. The first isconceptually, making use of the organizational scheme built into whatever software isbeing used. The second is physically, in terms of the devices and media used to storethe files.

All personal computers have an operating system, the basic software that runs thecomputer and allows other software packages to be used. One important function ofthe operating system is to provide an organizational scheme for files. Theseorganizational schemes often use physical analogies, such as branching trees or groupsof nested folders. In any case, the basic idea tends to be the same: related files aregrouped together, then related groups are grouped together, and so forth. Softwarepackages generally make use of the operating system's organizational scheme. Somesoftware packages also have their own organizational schemes.

Although it clearly makes sense to group related files together, it is not always clearwhat "related" means. For example, does it make sense to group all scanned air photostogether and all maps together, or should files covering a specific city or area begrouped together regardless of their source? Is it enough to group satellite imagestogether, or should separate sub-groups be created based on the dates the images wereacquired? Should city maps be grouped together on the basis of scale, or does that levelof classification add complexity without adding utility?

As with naming files, there is bound to be a certain amount of experimentation. Theunderlying ideas may seem almost too basic and commonplace to merit discussion, butas with any filing system, it is worth thinking it through carefully, preferably before alarge body of data has been accumulated.

This same principle applies to the physical organization of files. There are manydifferent kinds of storage media. They vary both in type and in capacity. For example,depending on funding, equipment, size of data files, and the availability of supplies,files can be stored on floppy disks, on-line mass storage devices, removable massstorage devices, tapes, or cartridges, to mention just some of the options. Types ofstorage media vary greatly in capacity, access speed, cost, and ease of use.

The most general rule is to store related data together on a single unit. Because spatialdata so often results in very large files, floppy disks are limited in their usefulness, andunits that have a larger capacity, such as hard disks and cartridges, are much moredesirable.

Simple Computer Mapping and Imaging /125

Making backups of digital files. A backup is a copy of a file, stored away in casesomething happens to the original. All data should be backed up and the backup copyshould be stored separately from the original. Why should this type of precaution betaken? Because spatial digital files are valuable. It can take considerable time and effortto create them, and there are many ways in which they can be either damaged ordestroyed. Human error is the most common reason data gets lost. Other sources ofdata loss include power surges because of weather or problems with the power source,storage media wearing out, and equipment getting damaged.

Backups should be made frequently as new files are created and existing files aremodified. Some users backup several times a day and others do so only once a week.When deciding on a reasonable backup frequency, the time and effort required to makea backup should be balanced against the time it would take to duplicate material if itwere lost (always assuming that the files can be duplicated).

Many users find it useful to keep more than one generation of backups; that is, when abackup is made, the previous one (or sometimes even more) is kept. In this case, it isvery important to label the backups carefully to avoid confusion. Including the date thebackup is made as part of the file (or folder or directory) name is one way of keepingthings straight.

One aspect of making backups that deserves special consideration is how quickly andeasily they can be made. If backups are difficult to make or if they take what is seen asan unreasonably long time, they often will not be done. Even when users are wellaware of the importance of backups and are (theoretically) committed to making them,convenience often wins out and backing up is put off until later. Unfortunately, "later"sometimes turns out to be right after an accidental loss of data. Whenever possible,both the software and hardware used for backing up should be chosen for ease andspeed of use. Ideally, backup equipment should be included as an integral part of acomputing system (and budgeted for as such), rather than added on as an afterthought.

Archiving old files. As a database grows, new material is added and older material isgradually used less and less. When it is reasonably clear that files are not being used,they can be removed and stored separately from the database. This process is calledarchiving. Archiving helps keep the size of the database manageable without actuallydiscarding data. If an archived file is needed after all, it can be retrieved and eitherused on a one-time basis or returned to the database.

The same software and hardware used for backing up can be used for archiving. It isuseful to review a database periodically and decide whether it can be improved byarchiving unused files.

126/ Appendix B: Taking Care of a Digital Database

Some useful indexes. An index is a systematic list giving the location of, and oftenother information about, the items listed. It is useful to have one or more indexes,printed on paper, to help locate various components and products of a digital spatialdatabase. Several possible indexes are discussed here; depending on how the databaseis used, others may be worthwhile.

An overall or suimmary index lists all the files in the database and indicates where each isstored. The index might also include information about each file, such as whether it isan individual scanned image, a digital mosaic, a subscene of an image, or a computermap, as well as things such as file format and size.

A mass storage index shows what is stored on a particular unit. If, for example, maps orimages have been scanned and stored on cartridges, each cartridge should have anindex listing what is stored on it. At the very least, the index should be a list of filenames. It might also include, for each file, the source of the data (map, air photo, and soforth), the scanning resolution, the file format, and the file size. Mass storage indexesshould be created both for units that contain the active database and for those that storearchived files.

A print index relates printed output to the digital files that it came from. When a set ofmaps or images has been printed, an index should be created for that set, naming eachprinted map or image and the file it was printed from. The index might also indicatewhether the output is the entire file or a subscene of it and what software was used toprint it.

Delineation indexes were discussed in chapter 2. An air photo delineation index showsthe relative position of air photos in a scanned set. This type of index is very importantduring the scanning process; it is also used in creating digital mosaics. It is differentfrom the other indexes in that it is a graphic, rather than a list. A similar graphicdelineation index can be created for a digital mosaic of air photos. A graphic mosaicindex shows the location and extent of the air photo mosaic by superimposing theboundaries of the mosaic on a map of the area.

The indexes discussed here are examples of what is possible and useful. Every databaseis different in its content, structure, and use. The specific indexes that are needed willvary somewhat, also. What remains constant is the need for some sort of printed guideto the digital data.

Appendix CSatellite DataIn the context of mapping and spatial analysis, the term satellite data refers toinformation about the earth's surface measured by sensors mounted on satellitesorbiting the earth. The sensors measure reflectance from the earth's surface and sendthat information, in digital form, to receiving stations on the ground. Satellite data canbe used to create images of the earth's surface. These images can be used to createimage maps, base maps, and thematic maps; to identify and analyze features; and totrack certain types of change.

Satellite data in digital and image form are available commercially. SPOT data (from aFrench spacecraft) include multispectral (or false color) data in green, red, and nearinfrared bands at 20-meter-by-20-meter resolution and panchromatic or black-and-white data at 10-meter-by-10-meter resolution. It is also possible to purchase orthoSPOT data, stereo SPOT data, and SPOT DEM (Digital Elevation Model) data. Orthodata have been processed in order to provide geometrically corrected images that canbe registered to a coordinate projection system. Stereo data are in the form of pairs ofimages that, when printed and viewed through a stereoscope, give a three-dimensionaleffect. SPOT DEM data are generated by computer programs using the stereo data tocreate altitude matrices-digital descriptions of the earth's topography.

Two types of LANDSAT data (from the U.S. NASA LANDSAT satellites) are available.LANDSAT TM (Thematic Mapper) data are available at 30-meter resolution andLANDSAT MSS (Multi-Spectral Scanner) data are available at 56-meter-by-79-meterresolution. Other data are available at coarser resolutions. For example, U.S. NOAAAVHRR data are available at 1-kilometer or at 4-kilometer resolution.

Because of their resolutions, these types of satellite data are best for regionalinformation and have limited use in urban contexts. Most urban features, such asbuildings and streets, are too small to resolve well, even with 10-meter resolution.Although features that are smaller than a pixel will at times show up in an image,between 10 and 100 pixels are needed to depict a feature and its general shape with anydegree of reliability. For example, the LANDSAT TM resolution of 30 meters by 30meters was chosen to provide information on the average North American agriculturalfield, which is about 200 meters by 200 meters. In spite of these limitations, satellitedata, when used with other sources of information, can be valuable for tasks such ascreating base maps, detecting environmental change, and mapping urban growth.

To date, digital satellite data have generally been available on tape. The data have beenin binary form, usually prefaced by other information about the data. Before being used

127

128/ Appendix C: Satellite Data

for desktop mapping applications such as those described in this publication, such datahave to be transferred from tape to disk. Depending on the software being used, aformat translation is generally required as well. Both media and format translationsare, as a rule, not done by the end user. Such conversion requires both a significantinvestment in equipment, and individuals with format conversion computerprogramming expertise. However, as personal computers become more common,satellite data that can be used directly on personal computers are increasingly availablefrom original suppliers of satellite data (such as SPOT) and from third parties.

When satellite data are available in image form as a photographic print, the image canbe scanned using the technique described for air photos. When transparencies andslides are available, they may also be scanned when appropriate equipment, such as a

slide scanner, is available. The resulting scanned image can then be processed, printed,and viewed and used as part of the spatial database.

Appendix DThe Hardware and Software Usedin Developing the TechniquesThroughout this publication, we have discussed hardware and software in generalterms, rather than naming specific products. Change is both pervasive and rapid in thecomputer world-new products appear, old products disappear, and existing productschange constantly. If the techniques had been described in terms of individualproducts, much of the discussion would become out-of-date quite rapidly. The generalapproaches described remain valid longer than do procedures unique to specifichardware or software.

Other reasons for generality include the tremendous diversity of products available andthe great range of costs associated with them. Both hardware and software vary inpower, features, ease of use, reliability, and appeal to specific users, as well as in cost. Itis impossible to specify a "best" configuration of hardware and software; the choice ofwhich products to use depends on the funds available, the preferences of the specificusers, the equipment-if any-already in use, and the types of tasks to beaccomplished. Referring to the hardware and software in generic terms makes thepublication useful to a much wider range of potential users than would a product-specific discussion.

However, the techniques presented here were developed with specific, rather thangeneric, hardware and software, and the authors have frequently been asked to describethat specific equipment. This appendix lists the hardware and software that was usedand gives names and addresses of the manufacturers or distributors. This list is not anendorsement of any sort and is presented solely as information. It should beemphasized that, while care has been taken to provide accurate information, anyspecific or current information must be obtained from the manufacturers or theirauthorized sales representatives. This appendix is not intended to guide individuals orinstitutions in their purchases, but to provide information that is frequently requested.

The personal computers used were all members of the Apple Macintosh II family. Theoverriding consideration in selecting both hardware and software was the potential forthese tools to be used effectively in a resource-poor environment under harsh operatingconditions. For example, electrical power tends to be intermittent and surge prone.Environmental controls, such as cooling and dust controls, are limited or absent. AppleMacintosh personal computing hardware met the following criteria:

a) Low cost-both low absolute cost and value delivered for cost.

129

130/ Appendix D: Hardware and Software

b) Versatility-as a way of maximizing value. For instance, a printer capable of

producing both text and graphics is preferable to one capable of text only.

c) Dependability-a record of infrequent breakdowns and requiring minimum

maintenance. It is important that local personnel be able to do periodic maintenance

and perform basic troubleshooting without lengthy training.

d) User friendliness-can be learned quickly and used easily. Ideally, local users

will not only quickly learn and use the equipment, but soon be able to train others.

e) Tolerance of varying power sources and standards-must be surge tolerant. The

ability to run on an uninterrupted power supply (i.e., on battery power) is also a

plus.

Macintosh computers are manufactured by:

Apple Computer, Inc.20525 Mariani AvenueCupertino, CA 95014U.S.A.(408) 996-1010

The scanners used were the 16-graytone Apple Scanner and the full-color Abaton Scan

300/Color scanner. The Apple Scanner is no longer available and has been replaced by

the Apple OneScanner, which is capable of 256 graytones. The Apple OneScanner is

manufactured by:


The Abaton Scan 300/Color scanner is manufactured by:

Abaton (Subsidiary of Everex Systems, Inc.)48431 Milmont DriveFremont, CA 94538U.S.A.(415) 683-2226

In both cases, the scanning software required (described on page 131) was supplied

with the scanning hardware.

Simple Computer Mapping and Imaging /131

The mass storage device used was the Bernoulli Transportable 44 cartridge subsystemmanufactured by:

Iomega Corporation1821 West 4000 SouthRoy, UT 84067U.S.A.(800) 456-5522

At the time this publication was prepared, this particular mass storage system (using44MB cartridges) was being replaced with one using 90MB cartridges.

The printers used were members of the Apple LaserWriter H family, which ismanufactured by:


The system software used was Apple system software, which is provided with thecomputers.

The scanning software used with the Apple Scanner was AppleScan, which wasprovided with the scanner and which is not available separately. The scanning softwareused with the Abaton Scan 300/Color scanner was Adobe Photoshop with the AbatonPlug module, which is provided with the hardware. The Photoshop application, whichincludes image processing capabilities, is available from:

Adobe Systems, Inc.1585 Charleston RoadMountain View, CA 94039-7900U.S.A.(415) 961-0911

The drawing software used was MacDraw II and MacDraw Pro. Both are availablefrom:

CLARIS Corp.5201 Patrick Henry Drive, M/S C-111, Box 58168Santa Clara, CA 95052-8168U.S.A.(408) 727-8227

132/ Appendix D: Hardware and Software

The image processing software used was MAP II Map Processor: A Geographic

Information System for the Macintosh. Academic users can obtain it from:

John Wiley & Sons, Inc.605 Third AvenueNew York, NY 10158-0012U.S.A.

Nonacademic users can obtain it from:

ThinkSpace, Inc.316 Cheapside St.London, Ont. N6A 2A6Canada

Apple, Apple OneScanner, LaserWriter, and Macintosh are registered trademarks of

Apple Computer, Inc.

Abaton Scan 300/Color and Abaton PS Plug are trademarks of Abaton Technology, a

subsidiary of Everex Systems, Inc.

Bernoulli is a registered trademark of Iomega Corporation. Bernoulli Transportable is a

trademark of Iomega Corporation.

AppleScan was Developed using MacAppTM, MacApp© 1985, 1986, 1987, 1988. Apple

Computer, Inc.

MacApp is a trademark of Apple Computer, Inc.

MacDraw is a trademark of Claris Corporation.

Adobe Photoshop is a trademark of Adobe Systems, Inc.

MAP II Map Processor is a copyright of John Wiley & Sons, Inc.

Appendix ESelected Readings

This list is drawn from relatively recent books or revised editions in the areas ofgeographic information systems (GIS), spatial analysis, remote sensing, digital imageprocessing, cartography, automated cartography, graphic design, and field techniques.

Avery, Thomas E., and Graydon L. Berlin. Fundamentals of Remote Sensing and AirphotoInterpretation. 5th ed. New York: Macmillan, 1992.

This well-known work, formerly entitled Interpretation of Aerial Photographs, is amagnificently illustrated, high-quality reference on the subject. Recommended.

Burrough, Peter A. Principles of Geographic Information Systemsfor Land ResourcesAssessment. Monographs on Soil and Resources Survey No. 12. Oxford, U.K.: OxfordUniversity Press, 1986.

A good and affordable GIS reference book. Substantial material has becomeobsolete, however.

Clarke, Keith C. Analytical and Computer Cartography. Englewood Cliffs, NJ: PrenticeHall, 1990.

A modem book on automated cartography. Includes C code.

Curran, Paul J. Principles of Remote Sensing. London: Longman, 1985.

A thoroughly illustrated professional reference book.

Hum, Jeff. GPS: A Guide to the Next Utility. Sunnyvale, CA: Trimble Navigation, 1989.

A small (76-page) graphic booklet explaining the satellite based Global PositioningSystem (GPS) in a layperson's terms.

133

134/ Appendix E: Selected Readings

Jensen, R. John. Introductory Digital Image Processing: A Remote Sensing Perspective.Englewood Cliffs, NJ: Prentice Hall, 1986.

A solid and accessible treatment of this specific topic.

Lounsbury, John F., and F. T. Aldrich. Introduction to Geographic Field Methods andTechniques. 2d ed. Columbus, OH: Charles E. Merryl, 1986.

Takes a traditional approach to basic mapping techniques and instruments. Alsotouches on field questionnaires, sampling, and research design. Lacks material oncomputer-assisted fieldwork.

Maguire, David J., Michael F. Goodchild, and David W. Rhind, eds. GeographicalInformation Systems: Principles and Applications. London: Longman, 1991.

A substantial reference made up of 56 contributions. Also quite expensive aboutUS$300. Relatively strong on vector and thin on raster/imaging.

Muehrcke, Phillip C. Map Use: Reading, Analysis and Interpretation. 2d ed. Madison, WI:JP Publications, 1986.

An excellent reference on maps.

GIS WORLD, published by GIS World, Inc.GIS EUROPE, owned by GIS World, Inc..

These magazines are major sources of timely GIS information. GIS WORLD ispublished 12 times per year by GIS World, Inc., 155 E. Boardwalk Drive, Suite 250,Fort Collins, CO 80525, USA. GIS EUROPE is published 10 times a year byLongman Geolnformation, 307 Cambridge Science Park, Milton Road, Cambridge,CB4 4ZD, UNITED KINGDOM.

Ritchie, William, Michael Wood, Robert Wright, and David Tait. Surveying and Mappingfor Field Scientists. London: Longman, 1988.

Emphasizes conventional surveying and photogrammetric techniques.


Star, Jeffery, and John Estes. Geographic Information Systems: An Introduction. EnglewoodCliffs, NJ: Prentice Hall, 1990.

Useful as a reference book for remote sensing, image processing, and GIS. Sparse ingraphics.

Tomlin, C. Dana. Geographic Information Systems and Cartographic Modeling. EnglewoodCliffs, NJ: Prentice Hall, 1990.

Presents a theoretical and systematic modem approach to spatial processingbased on digital image processing. The emphasis is on theory rather than onpractical applications.

Tufte, Edward R. Envisioning Information. Cheshire, CT: Graphics Press, 1990.

This book on data graphic design presents many useful ideas; however, it is not verycomputer oriented.

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