The GIS Files

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The GIS files

v2.0 Mar 2003 © Crown copyright

The GIS files



The GIS files


Responsibility for this document

Danny Hyam, Ordnance Survey Technical Promotions Media Manager, is responsible for the

content of this document.

Change history

Version Date Summary of change

1.0 May 2002 First issue

1.1 Oct 2002 Minor change

2.0 Mar 2003 Second issue

Content

This document consists of 89 pages.

Distribution

The data file for this document is archived by Corporate Publishing as D01100.doc.

Approval for issue

Issued by Danny Hyam.

Trademarks

Ordnance Survey, the OS Symbol, OSGB36, Land-Line, Strategi, OSCAR and OSCAR

Route-Manager are registered trademarks and ADDRESS-POINT, Boundary-Line,

Get-a-map, Digital National Framework, DNF, Meridian, MiniScale, OS and OS MasterMap

are trademarks of Ordnance Survey, the national mapping agency of Great Britain.



The GIS files


Contents

Section Page no

1 Getting to grips with GIS...............................................................................................7 1.1 Introduction ....................................................................................................... 7 1.2 In the beginning.... there were maps.................................................................7

1.2.1 Map types.............................................................................................7 1.2.2 Map features ........................................................................................ 8 1.2.3 Map information ................................................................................... 8

1.3 Introducing raster and vector ............................................................................ 9 1.3.1 Maps in bits ..........................................................................................9 1.3.2 Vector data.........................................................................................10 1.3.3 Raster data.........................................................................................11 1.3.4 Vector v Raster .................................................................................. 11 1.3.5 Raster can be intelligent.....................................................................12

1.4 GIS = software + data.... a formula for success.............................................. 12 1.4.1 Building blocks ................................................................................... 12 1.4.2 GIS evolution......................................................................................13 1.5 Different types of GIS data..............................................................................14 1.5.1 Data sources...................................................................................... 14 1.5.2 Ordnance Survey data and GIS .........................................................15 1.5.3 Linking data........................................................................................16

1.6 The significance of scale.................................................................................17 1.6.1 Scale basics ....................................................................................... 17 1.6.2 Scale of capture ................................................................................. 17 1.6.3 Generalisation .................................................................................... 17 1.6.4 Be careful with scale .......................................................................... 19

2 Geographical data ...................................................................................................... 20 2.1

Introduction ..................................................................................................... 20

2.2 Data capture from maps..................................................................................20

2.2.1 Scanning ............................................................................................21 2.2.2 Digitising.............................................................................................21 2.2.3 Vectorisation ...................................................................................... 22

2.3 Surveying and remote sensing........................................................................22 2.3.1 Early surveying techniques ................................................................22 2.3.2 Photogrammetry – remote sensing.................................................... 22 2.3.3 The Global Positioning System (GPS) ............................................... 23 2.3.4 Pen computers................................................................................... 24

2.4 Position matters ..............................................................................................24 2.4.1 Georeferencing .................................................................................. 24 2.4.2 Coordinate systems ........................................................................... 25 2.4.3 Methods of referencing data ..............................................................25 2.4.4 The third dimension: height................................................................26 2.4.5 Global, regional and national systems ............................................... 27

2.5 What does GIS data look like?........................................................................27 2.5.1 Styles .................................................................................................27 2.5.2 Changing the appearance of vector data ........................................... 28 2.5.3 Changing the appearance of raster data............................................28

2.6 Looking at multiple layers................................................................................29 2.6.1 Combining layers ............................................................................... 29 2.6.2 Identifying change over time ..............................................................30



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2.7 The third dimension.........................................................................................30 2.7.1 From 2-D to 3-D ................................................................................. 30 2.7.2 3-D GIS data ...................................................................................... 31 2.7.3 Realistic models ................................................................................. 32

2.8 Topology. It is all about relationships ..............................................................34 2.8.1 It is all about relationships..................................................................34 2.8.2 Link-node topology.............................................................................35 2.8.3 Polygon topology................................................................................35

3 Adding real-world information..................................................................................... 37 3.1 Introduction ..................................................................................................... 37 3.2 The attributes of map features ........................................................................ 37

3.2.1 What are attributes?...........................................................................37 3.3 The attributes of map features ........................................................................ 38

3.3.1 Attribute tables ................................................................................... 38 3.4 GIS can tell you everything worth knowing about anything.............................39 3.5 Using GIS? Be selective ................................................................................. 40 3.6 Geocoding.......................................................................................................41 3.7 Structured GIS data is the key ........................................................................ 42

3.7.1 The significance of structure ..............................................................42 3.7.2 Address data ...................................................................................... 43 3.7.3 Road network data ............................................................................. 43 3.7.4 Spaghetti data .................................................................................... 45 3.7.5 Polygon structured data ..................................................................... 45

4 Putting it all together as a system...............................................................................47 4.1 Introduction ..................................................................................................... 47 4.2 Unlocking the information................................................................................47 4.3 GIS reveals all.................................................................................................48

4.3.1 The thematic map .............................................................................. 48 4.3.2 Visual analysis ................................................................................... 49

4.4 What happens where? – The power of the spatial query................................50 4.4.1 Basic spatial querying ........................................................................ 50 4.4.2 Buffers................................................................................................51 4.4.3 Overlay operations............................................................................. 51 4.4.4 The tricky but important bit.................................................................52

4.5 Show me the way to go...................................................................................53 4.5.1 Network analysis................................................................................ 53 4.5.2 In-car navigation.................................................................................54 4.5.3 Drive-time analysis.............................................................................54 4.5.4 Optimum-path analysis ......................................................................55

4.6 Some simple GIS examples............................................................................55 4.6.1 How many useful applications can GIS provide?...............................55 4.6.2 Flood risk............................................................................................56 4.6.3 Emergency services...........................................................................56 4.6.4 Estate agents ..................................................................................... 56 4.6.5 Nature conservation........................................................................... 57 4.6.6 Retail ..................................................................................................57 4.6.7 3-D environmental impact analysis .................................................... 57 4.6.8 Airport-noise pollution ........................................................................ 58

5 Case studies............................................................................................................... 59 6 Chapter 6 – Expert GIS concepts............................................................................... 60

6.1 Introduction ..................................................................................................... 60 6.2

Data formats....................................................................................................60



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6.2.1 GI data compared to other data ......................................................... 60 6.2.2 Proprietary file types .......................................................................... 61 6.2.3 Translators and transfer formats ........................................................ 62 6.2.4 Open formats ..................................................................................... 64

6.3 Standards........................................................................................................65 6.3.1 Metadata ............................................................................................65 6.3.2 Gazetteers..........................................................................................66 6.3.3 Gazetteers in England and Wales......................................................67 6.3.4 XML and GML .................................................................................... 67 6.3.5 Future standards................................................................................ 68

6.4 Spatial databases............................................................................................69 6.4.1 Database fundamentals ..................................................................... 69 6.4.2 Relational databases..........................................................................70 6.4.3 Object databases ............................................................................... 72 6.4.4 GIS and databases ............................................................................ 73 6.4.5 Advanced database technology.........................................................74

6.5 Derived mapping.............................................................................................75 6.5.1 Generalisation .................................................................................... 75 6.5.2 Text placement...................................................................................76 6.5.3 Automated cartography......................................................................77 6.5.4 Data from imagery..............................................................................78

6.6 Web GIS..........................................................................................................80 6.6.1 Simple maps in web pages ................................................................80 6.6.2 Internet mapping sites........................................................................81 6.6.3 Internet GIS software ......................................................................... 81 6.6.4 Web GIS futures.................................................................................82

6.7 Mobile GIS ...................................................................................................... 84 6.7.1 Positioning..........................................................................................84 6.7.2 Location-based services (LBS) .......................................................... 85 6.7.3 Personal and vehicle navigation ........................................................86 6.7.4 LBS for the mass market....................................................................87 6.7.5 Telematics..........................................................................................88



The GIS files




The GIS files


1 Getting to grips with GIS

1.1 Introduction

In this, the first chapter of The GIS files, you will find out about the fundamental building

blocks of geographical information systems (GIS), how information that has traditionally been

shown on maps can be converted into computerised form.

In the 1930s and 40s geographical analysis was conducted by overlaying different types of

maps of the same area. Since the 1950s systems have evolved to convert this mapping into

digital form and more recently to use this data for analysis and problem solving. Nowadays

GIS is everywhere; you may even have some GIS software on your PC without even

knowing it is there!

1.2 In the beginning.... there were maps

1.2.1 Map types

The story of GIS begins in the world of maps. A map is a simplified visual representation of

real things from the real world.

Maps can model the world in more than one way:

A Topographic map shows the physical surface features, for example, roads, rivers,

buildings.



The GIS files


A Contour map shows lines which connect point locations at which a certain property has

the same value, for example, height above sea level, isobars showing air pressure.

A Choropleth map shows areas characterised by some general common feature, for

example, political maps, agricultural crop types.

1.2.2 Map features

The earliest GIS programmes were developed simply to allow map information to be stored

in computerised form. This made maps easier to store, reproduce and update. The process

of capturing map information in digital form begins with the classification of all features.

Look at any map – the different shapes and symbols are used to illustrate features. There

are four main types of symbol used to depict the different feature types. In fact, all map

features can be divided into one of four different categories:

•

Point (for example, a cross symbol to represent achurch).

• Line (for example, a yellow line to represent a road).

• Polygon shape or area (for example, a blue area to

represent a lake).

• Text (for example, the name of a building).

The map is actually a very sophisticated information source. The human eye is able tointerpret a rich amount of information from a map simply from the pictorial content. This is

enhanced by the use of textual annotations (names of objects are written on a map in such a

way that the letters do not get in the way of the geographical features themselves). GIS

works by taking all of this information and recording it in electronic form.

1.2.3 Map information

How is map information translated into digital form and read by a computer?

The GIS must be able to store information about:

• The geometry: the shape and location of the objects.

• The attributes: the descriptive information known about the objects, normally displayed

on a map through symbology and annotation.



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There are two fundamental methods of storing this map information in digital form, raster and

vector . These are covered in section 1.3 Introducing raster and vector so hit Next to move

on.

1.3 Introducing raster and vector

1.3.1 Maps in bits

The first step in converting map information into a form that can be read by a computer is to

describe the shapes and locations of features using a series of numbers. Computers store

information in sequences of binary digits (bits), which form a code for every possible number

or letter.

This fits with the way maps reference geographical locations on the earth’s surface, through

a system of coordinates. These coordinate systems can be local, national or international.

Look at an Ordnance Survey map and you will notice, along the sides, there are a series of

numbers associated with a grid covering the whole map area.



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These numbers refer to coordinates from the British National Grid. All locations and shapes

can be defined in terms of x and y coordinates from a given grid system: it is these numerical

values which are used to translate map information into digital form. This applies in both

vector and raster formats.

1.3.2 Vector data

In vector data the features are recorded one by one, with shape being defined by the

numerical values of the pairs of xy coordinates.

A point is defined by a single pair of coordinate values.

A line is defined by a sequence of coordinate pairs defining the points through which the line

is drawn.

An area is defined in a similar way, only with the first and last points joined to make a

complete enclosure.

Vector data can be thought of as a list of values.

In the example above the map represents a building as a simple red rectangle. In vector data

the position and shape of the building is captured as a series of four pairs of numerical

coordinates. To reproduce the building in a GIS the computer reads these values and draws

a line linking the coordinate positions.

The vector version can also store additional context information about these features – the

attributes – a very important aspect, which will be explained in later chapters of the GIS files.



The GIS files


1.3.3 Raster data

In raster data the entire area of the map is subdivided into a grid of tiny cells. A value is

stored in each of these cells to represent the nature of whatever is present at the

corresponding location on the ground. Raster data can be thought of as a matrix of values.

The major use of raster data involves storing map information as digital images, in which the

cell values relate to the pixel colours of the image. In the example above the data records the

colour of the feature which occupies that part of the map surface; the values recorded in the

cells are either white, blue or red. To reproduce the image the computer reads each of these

cell values one by one and applies them to the pixels on the screen.

1.3.4 Vector v Raster

Both types of data are very useful, but there are important differences – the characteristics

below are broad generalisations which do not necessarily apply in all circumstances.

Vector relatively low data volume faster

display can also store attributes less

pleasing to the eye does not dictate how

features should look in the GIS.

Raster relatively high data volume slower

display has no attribute information more

pleasing to the eye inherently stores how

features should look in the GIS.

Above is a simple comparison between vector and raster. What do you notice about the

images as you zoom in and out?



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In this example the raster data looks nicer but, as you zoom in, the pixel structure becomes

obvious. Eventually the image looks like a piece of modern art rather than a detail of a map!

The definition of the features is dependent upon the size of the individual grid cells – the

resolution.

The vector data is more like a graph with a line drawn between points, the width staying the

same however close you zoom.

1.3.5 Raster can be intelligent

In general terms, vector data is more valuable to GIS systems because it can store a large

amount of information about the features. Raster data can store the colour values that make

up the whole image of an area and can be used in GIS as unintelligent backdrop mapping,

like the map image on the previous page, or data from other sources like aerial photographs.

However, in some more specialist applications, raster data can be used to do more than just

capture a visual image. The idea of storing a matrix of values across an area is particularly

suited to recording measurements of a continuous nature.

For example, archaeologists will often scan an archaeological site with sensors or probes to

get a grid of magnetic or electrical readings that may reveal patterns suggesting the

presence of structures under the soil (see example above).

This is just one example of a scientific use of raster data, and in fact, such images can also

be loaded into a GIS for analysis alongside map information.

The next section, GIS = software + data, looks at how people bring data together with

computer software to build useful systems.

1.4 GIS = software + data.... a formula for success

1.4.1 Building blocks

Any successful example of GIS is based on two fundamental components:

• the map data; and

• the computer software to perform calculations and analysis.



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There are many different organisations producing data for use in GIS; Ordnance Survey is

just one of these. There is also a large industry in GIS software with hundreds of companies

producing thousands of products. To find out more about some of these companies visit the

Licensed Partner area.

To be a truly effective system, a GIS needs good software and good data. One without the

other will not be productive. The other vital component is people. The GIS will only provideuseful answers to problems if the user is able to ask the right questions and can interpret the

results.

A GIS can be a simple desktop software package costing a few hundred pounds, running on

a standalone PC. Alternatively a single system can involve a very large network of

workstations and servers with many different software components costing millions of

pounds.

1.4.2 GIS evolution

GIS packages have evolved from a combination of two well established types of software:

the way in which map geometry is handled is based on graphics and computer-aided-design

(CAD) technology; the way in which attribute information is handled has been developed

from conventional spreadsheet and database technology.

A professional GIS user must be able to understand the disciplines of both these types of

software, as well as appreciating geographic principles.

http://www.ordnancesurvey.co.uk/partners/licensed_partners/intro.cfm





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One of the historical drawbacks to GIS has been the high cost involved. Nowadays reduced

costs are making GIS more accessible. The Internet is also playing a big part in increasing

the extent to which GIS technology is being utilised. Many web sites use some underlying

GIS processing to present customised map images to your browser.

Section 1.5 Different types of GIS data, looks at the different types of information which can

be used.

1.5 Different types of GIS data

1.5.1 Data sources

The most common form of GIS data is based on topographic features, that is, the features

that make up the physical structure of the land surface. Topography includes the relief of an

area (the shape of its surface) and the position of both natural and man-made features.

In addition to topographical data, there are more diverse sources of information that can belinked into a GIS.

Large amounts of data relating to both people and the environment can be viewed and

analysed in a GIS. The image above shows how a layer of environmental information can be

overlaid on a map backdrop.

Even aerial and satellite imagery can be incorporated into a GIS and

viewed along with other data for the same area, as long as the

ground extent of the image can be identified. The most powerful GIS

applications use data taken from a range of different sources.



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1.5.2 Ordnance Survey data and GIS

Ordnance Survey produces many different GIS data products, the diversity of these products

is itself an indication of the many different ways in which GIS can be used. They range from

simple raster images of road atlas style mapping, to very detailed vector data extracted from

the National Topographic Database, which is Great Britain’s official archive of large-scale

mapping. This database is incredibly detailed – it shows every house, every fence and everystream in every single part of Great Britain.

Zoom through the samples below to get an idea of the range of different types of product

available for GIS use.

MiniScale™ Boundary-Line™ Strategi®

1:250 000 Scale Colour

Raster Meridian™ OSCAR® Route-Manager

1:50 000 Scale Colour Raster Land-Line® OS MasterMap™



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1.5.3 Linking data

Here you can see an example of ADDRESS-POINT™ data. This is far removed from the map

model of depicting information, this illustration is meant to demonstrate that not all data in a

GIS will look like a map. At first glance the sea of dots may not appear particularly useful:

ADDRESS-POINT is not a cartographic product, but is designed to be used in conjunction

with other layers of information within a GIS application. Use the numbered buttons to seehow this type of data becomes useful.

ADDRESS-POINT can be used to identify specific properties against a map backdrop or to

link to other sources of associated information, like voting wards. Linking information in this

way is explained more fully in later section of the GIS files.

In the last section of this introductory chapter we look at how scale is important in GIS.



The GIS files


1.6 The significance of scale

1.6.1 Scale basics

When looking at a paper map, probably the most important thing to bear in mind is the map

scale. This is the relationship between the dimensions on the paper to the real distance onthe ground.

If a building is 13 m long in the real world and a map depicts this length as 13 mm, the scale

is 1:1000. Multiplying the distance on the map by the scale factor gives you the real world

dimension.

In the world of GIS and computerised mapping things are more complicated. A description of

scale can lose its meaning – the scale of the image on screen can depend on the monitor

size. The image above may appear 13 mm long on some screens but not others.

1.6.2 Scale of capture

All GIS packages enable you to zoom in and out on the map data as much as you like.

Again, this means that you cannot say that the map data has a particular scale. However, all

topographic data has a scale of capture, that is, the source data was captured at a particular

scale, whether this was a paper map or an aerial photo.

It is important to understand the source scale of your data for two fundamental reasons:

• data from a particular scale should only be viewed within a certain range of magnification

for it to make sense visually; and

• combining two or more datasets together is only appropriate if they have an equivalent

scale of capture.

1.6.3 Generalisation

Very detailed mapping, showing the outline of individual objects such as walls and fences, is

known as large-scale data. The positional accuracy of features shown on this type of

mapping is very high but there is so much detail that if you zoom out the view becomes very

cluttered.



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The mapping that most of us recognise has been deliberately simplified. A cartographer

creates these simple, readable maps by selecting information from a larger-scale source. Not

all the detail from the source map can be shown. For example, a road atlas that attempted to

show every building in the country would become far too cluttered, so some features are

aggregated, smoothed out or omitted altogether.

The illustration below shows how large-scale data when viewed at a small scale (zoomedout), appears cluttered, whereas small-scale data when viewed at a large scale (zoomed in),

appears very sparse.

Large-scale data Small-scale data

Sometimes it may be necessary to alter a feature’s true survey position slightly to make

space for the map symbols. Furthermore, the thick red lines of an A road are shown much

wider on the map than the actual road is on the ground. This science of small-scale map

production is known as generalisation.



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1.6.4 Be careful with scale

Many GIS data products are created from generalised map sources – they are very useful for

simple, quick to draw overview maps. However, you can view the data at any scale once it

has been incorporated into a GIS – as we have seen, this can lead to data no longer making

sense if you zoom in too closely. Worse still, the effects of generalisation will show up if this

data is viewed against other more large-scale mapping.

The demonstration above shows what can go wrong. The coloured lines are generalised map

data captured from a road atlas. When you zoom in, the deviation of the simplified features

from their survey position is apparent when the large-scale data becomes visible. The

generalised data is not wrong, it is just being magnified more than was ever intended.



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These principles may seem straightforward, but it is alarming how often the real benefits of

GIS are lost through using inappropriate combinations of data. For example, you may find an

accurate road layer being shown against a less accurate coastline, which can give the (false)

impression that the restaurant you are looking for is actually an underwater one!

Getting to grips with GIS has introduced some very fundamental principles of how map

information can be stored in GIS.

2 Geographical data

2.1 Introduction

In this chapter we go into more detail about the geographical data part of a GIS. We look at

the ways in which the geographical data (more correctly called geospatial data) can be

captured for GIS and then manipulated.

Geospatial data stores information about the location, shape and attributes of real objects.

So what? You might say; paper maps have been doing this for centuries. But it is the

capturing of this information in digital form that makes it much easier to store and reproduce.

It also enables the power of computers to be used in manipulating, updating and analysing

the information in many different ways.

Let us start by looking at how we capture data from maps, in section 2.2 Data capture from

maps.

2.2 Data capture from maps

With so many paper maps in existence, it is not surprising that a lot of geospatial data has

been created using them as a template. It is also possible to create geospatial data by taking

measurements direct from physical surveys. These days, most geographical information is

captured in digital form at the point of initial survey. But some data is still created by

converting paper maps into electronic form. The two most important methods are scanning

and digitising.



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2.2.1 Scanning

You are probably familiar with scanning technology

already, many home and office PCs come with a desktop

scanner. The scanner will take any printed image and

take a picture of it. By capturing the image in digital form

it can be stored on the computer and displayed on

screen. Scanning a map is a straightforward process and

generally fast, but it does not provide for the capture of

attribute information for features, such as the address of

a building. As discussed in Chapter 1, raster

data uses up a lot of disk space, so rasterisation of maps by scanning is not always the

most efficient method. However, it is very good for storing the cartographic style of the

map.

2.2.2 Digitising

Digitising requires the use of special equipment. The source map is laid flat on a table (tablet)

and an electronic cursor is passed over the features of the map. In this way, each of the

coordinate points which make up the different shapes can be identified. By clicking the cursor

when it is held over a point, digitising captures map data in vector form.

For digitising to work, the tablet must have a magnetic field embedded in the flat surface, so

as the cursor is moved around the map, its location can be identified.

Digitising can be very time consuming because every single point or vertex must be captured

individually. Ordnance Survey’s National Topographic Database currently contains more than

230 million features. You can imagine what a time consuming task it was to digitise it

originally. Fortunately, the database is maintained by surveying methods that generate digital

data directly.

When a cartographer is capturing information by digitising, it is possible to attach attribute

information to features. Often, the digitising tablet has some kind of menu of feature types.

Once a particular feature is digitised the resulting data contains information about its type

and shape.



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2.2.3 Vectorisation

Some very specialised computer systems are able to convert raster data to vector data by

recognising patterns in the image. For instance, it can guess that a sequence of coloured

pixels which seem to form a line across the image are showing a linear feature of some kind.

If the system knows the extent of the real position of the image, it can convert these shapes

into vector information. This vectorisation from raster data can be a fast method of capturebecause it can be automated, but is usually less accurate than manual digitising.

Now we will look at more direct methods of obtaining geospatial data, in section 2.3

Surveying and remote sensing.

2.3 Surveying and remote sensing

Surveying techniques have undergone a remarkable process of evolution. Modern survey

technology is extremely complex. Here we illustrate the most significant advances.

2.3.1 Early surveying techniques

In simple terms, the job of the surveyor is to measure the size,

shape and relative location of physical objects in the outside

world. Size and distance are fairly easy – you can use physical

measuring tools of stable and constant length to record these

dimensions. Some of the earliest long measurements were

made using glass rods end to end, to fix a distance between two

points on the ground. Such rudimentary methods are still in use

today.

From the earliest days of surveying, surveyors have exploited the rules of trigonometry todeduce distances between points on the ground without actually having to measure them

directly. Once you have accurately recorded the distance between two points, you can then

identify the distance to any third point by simply measuring the angles between all three. This

process is called triangulation and was the basis for Ordnance Survey’s original creation of

detailed mapping for the whole of Britain. The theodolite was the traditional optical tool used

to survey in this way, and more recently electrical devices were developed to conduct this

kind of ground measurement.

2.3.2 Photogrammetry – remote sensing

Photogrammetry is the science of measuring objects from photographs.Historically, this meant using aerial photographs to capture topographic

information. The first photogrammetric surveys were conducted more

than 100 years ago.

Now satellite pictures are also used to record the location and

geometry of features on the ground. Remote sensing is another term

describing the use of aerial and space imagery to record geographical

information. It includes the interpretation of other phenomena such as

vegetation type or land use shown in the Earth’s reflectiveness to

different wavelengths of electromagnetic radiation.



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Initially, maps were created from aerial photographs by various kinds of tracing mechanism.

Sophisticated devices were engineered to allow an operator to view and trace a pointer

around the visible features on the photograph. Using a system of wheels and pulleys, this

motion was mechanically reproduced by a drawing arm. Such machines used stereoscopic

viewing to survey in 3-D (three dimensions). As technology has advanced, the techniques of

digitising and scanning have become important aspects of photogrammetry.

Ordnance Survey has captured a significant amount of its detailed mapping by digitising fromaerial photographs. Remote sensing by satellite is now widely used for data capture and, as

the accuracy increases, this method could replace ground survey and aerial photos.

2.3.3 The Global Positioning System (GPS)

The development of the GPS, by the United States Department of Defense, is revolutionising

the world of surveying. It enables positioning of objects on or above the earth’s surface in an

absolute sense, not just in relation to other nearby features (as in the use of photogrammetry

described previously, in which locations are defined relative to the known position of certain

features in the image).

GPS can be used almost anywhere in the world, 24 hours a day, in all weathers. A

constellation of 24 satellites orbit the earth and send signals that can be picked up by GPS

receivers. GPS measurements are taken by computing the distance between the receiver

and the satellite. If a receiver picks up signals from four or more satellites, a 3-dimensional

position can be calculated. Certain methods can be used to increase the accuracy of the

position to the 1cm level, either in real time or afterwards during post-processing.

GPS measurements are obtained in the GPS coordinate system: World Geodetic System

1984 (WGS84). Users should be aware that this position usually needs to be converted into

the local coordinate system for the region, OSGB36®

in Great Britain, enabling GPS to be

used alongside the local mapping. GIS data collectors can make use of the free GPS service

provided to locate map objects and features directly in the field. Full details of this are

available on Ordnance Survey’s GPS web site (http://www.gps.gov.uk).

http://www.gps.gov.uk/





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Ordnance Survey uses GPS to locate map detail. However, there

are many other uses for GPS, such as navigating boats, planes

or cars, monitoring the stability of structures, and providing

location information for recreational users.

2.3.4 Pen computers

Whatever the method used for measuring the shape and location objects, modern surveyors

rarely record information by hand drawing detail on a master survey document (see surveyor on left of picture with large board). Instead, they use hand-held pen computers equipped with

flat, touch-sensitive screens (see surveyor on right of picture). These computers allow the

surveyor to draw and click directly onto the screen to update map information while out in the

field. Importantly, this means that new map features can be inputted directly as digital data.

Ordnance Survey surveyors use such a system known as PRISM, standing for Portable

Revision and Integrated Survey Module. It enables the coordinates of new objects to be

added in reference to the existing features. Features that have been demolished can be

deleted while text names can be added using a freehand character recognition facility. So

even if the geographical objects have been measured with the most rudimentary and

time-honoured techniques, a tape measure for instance, the information will still be recordedin electronic form out in the field.

Now we have looked at obtaining geospatial data, we need to look at how we relate it to the

world or to other datasets in section 2.4 Position matters.

2.4 Position matters

2.4.1 Georeferencing

Is your data georeferenced?

It is important to understand this concept because when you use a GIS, you are often

combining different layers of spatial data. An over riding coordinate system is needed so that

spatial data layers can be referenced to the earth’s surface in the same way. Otherwise, if

you use different coordinate systems, there will be no way to analyse the relationship

between the data.

In the image below, two layers of data have both been georeferenced to the same coordinate

system and hence match together.



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What is georeferenced data?

Georeferenced data is spatial data that is referenced to a location on the earth’s surface. To

do this, common frames of reference and coordinate systems have been set up. This is a

tricky business and this section will explain why.

2.4.2 Coordinate systems

It is clear that points and features on, above and below the earth’s surface have position. To

express that position, you need to choose an appropriate coordinate system so that positions

can be defined within it. Such a system must obviously give each point a different coordinate.

There are thousands of coordinate systems in use throughout the world. It is possible for a

user to invent their own and, indeed, this is often done for large engineering works. The

disadvantage is that points outside the area of the bespoke system cannot be coordinated

within it and the relationship between the bespoke and other systems is hard to define.

2.4.3 Methods of referencing data

There are two fundamental methods of referencing spatial data:

The first, known as the geo-centric system, uses a 3-D coordinate system with the centre of

the earth acting as the origin of the three axes. This method is universally used in scientific

applications, but it is user unfriendly when applied to points on the earth’s surface. This is

because the axes are made parallel to the spin axes of the earth rather than north (or some

other arbitrary direction). The system can be expressed in two ways: either as a

3-D Cartesian coordinate of the form x ,y ,z; or as latitude), longitude) and height (H) above a

known reference surface – the ellipsoid (see section 2.4.4 The third dimension: height ). It is

important to note that the x and y do not refer to east and west or north and south.



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The second, more common, system is the projection. This takes the 3-D coordinates and

expresses them as a plane plus height above it. In other words, it flattens out the curved

earth in a small region to a flat surface. Coordinates on the plane tell us where a particular

point within the projection is being used and are measured as distances, east and north from

the starting point or origin.

To avoid errors, the extent of the projection is usually limited to a small part of the earth’s

surface. Choice of projection is dependent on the area of the world being considered.

Different projections have different properties. Spatial data users choose the projection that

provides the least distortion, in distance, direction, scale, area and so on, within the region

being considered.

Bear in mind that there are dozens of different types of projection and each can have

hundreds of different definitions depending on where they are used. In Great Britain the

chosen projection is known as Ordnance Survey National Grid.

2.4.4 The third dimension: height

Height can be expressed in two ways:

The first, and by far the most obvious, is to measure heights from sea level. However, this

can be difficult as the sea level is irregular and constantly changing, making measurementsof height inland both complex and expensive.

The second was created specifically because of these drawbacks. Scientists invented a more

regular surface called an ellipsoid , which approximates to sea level. However, because the

shape of sea level is complex, hundreds of different ellipsoids are required depending on the

area of earth being modeled. Within Great Britain the ellipsoid of choice is know as Airy 1830

and this is used for the National Grid projection.



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2.4.5 Global, regional and national systems

Many national mapping agencies have defined local referencing systems to meet their

needs. In Britain, for example, we have the National Grid. These are perfectly acceptable

when working within a country, but problems exist in drawing up multinational data unlessthere is cross-border convergence. In Europe local coordinate projections are often referred

to the European Terrestrial Reference System 1989 (ETRS89) because it is fixed at a point

in time and well defined.

Probably the most common global coordinate system in use today is the GPS-based World

Geodetic System 1984 (WGS84). This is fixed to points on the earth’s surface which move

over time because of changes in the earth’s crust.

A significant global projection system is Universal Transverse Mercator (UTM). This is a

defined set of projections that cover the whole world and allow countries to share spatial data

more easily.

Once the digital data has been georeferenced, you can display the information in an infinite

number of ways. See section 2.5 What does GIS data look like?

2.5 What does GIS data look like?

2.5.1 Styles

The very simplest advantage that GIS gives in comparison

with paper maps is that you can change the appearance of the

information to any style you like. In conventional mapping a

large amount of time and effort is spent on deciding

appropriate colours and styles for the depiction of features, to

ensure that the image is as clear and informative as possible.

The flexibility of GIS adds an extra dimension to this process: you can change the

appearance depending on exactly what message you want to convey.



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2.5.2 Changing the appearance of vector data

The greatest flexibility comes when using vector data; remember that all the computer stores

is a set of coordinates which make up the shape of the object. Any GIS will let you choose

the colour and style of how the features are represented on screen. These styles will then be

reflected in the printed output from the system and, ironically, many organisations use GIS

simply to create customised printed maps.

With point features you can change the symbol type and the colour, and for line features you

can change the style and colour. For area features you can change the colour of the shape

itself as well as its perimeter. The colouring of the body of the shape (the fill ) can be made

solid, patterned or even transparent.

2.5.3 Changing the appearance of raster data

As explained in section 1.3 Maps in bits, the nature of raster data inherently defines how the

mapping should appear. The raster map is an image of coloured pixels, and the fact that a

road is depicted comes from a visual interpretation of adjoining pixels of the same colour, notfrom any information saying this is a road in the data structure itself. The only data entities

are the pixels themselves, and the only intelligence stored about those pixels is their colour.

Having said that, it is still possible to alter the appearance of raster data in a GIS. In simple

terms you could make all red pixels appear blue, for instance. Usually this is not a good idea,

because the image was designed with the most appropriate colours in the first place.

However, in some circumstances this facility is useful because you can tone down the colour

scheme of the raster image to allow other information to be placed on top and made more

readable. The graphic below demonstrates some different ways in which the appearance of a

raster image can be adjusted.

The point made above – the fact that you can place a layer of text information on top of a

separate layer of raster data – leads us into the next section. In section 2.6 Looking at

multiple layers, we will look at how different data layers can work together.



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2.6 Looking at multiple layers

2.6.1 Combining layers

GIS really gets going as a powerful tool when you start to work with different layers of

information about the same geographical area at the same time. When compiling aconventional map, the cartographer has to draw a balance between

displaying as much information as possible to make the map useful without

adding so much detail that it becomes cluttered and confusing. With GIS,

this problem is removed – many different layers of information can be

added, and shown in different combinations and in a different order,

depending on the particular message to be conveyed. Using the power and

flexibility of the computer, different data layers can be switched on and off

at the click of a mouse, so that many different views can be created for the

same location.

The image below represents the way in which a GIS can display many different layers of information at the same time. Using different combinations, the display can serve a wide

range of purposes that could only otherwise be achieved by producing a whole set of

different paper maps.

Referring to the graphic: step 1 creates a communication network map by switching on just

the towns, roads and railways; step 2 generates a view of the relief of the area by switching

on the contours and rivers; and step 3 gives a view of all these layers together which can

help to analyse the spread of urbanisation in relation to networks and relief.

1) Communicationnetwork map

2) Relief map 3) Urban growth

Vector data can also be shown in combination with raster data, the latter usually in the form

of a backdrop.



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2.6.2 Identifying change over time

Another benefit of mixing and matching different layers is that by combining mapping for the

same area surveyed at different times, you can identify any changes. The example below

shows the changes that have occurred as the coast has been significantly eroded over time.

The green line is the current position of the cliff.

These are just simple examples of how a different message can be portrayed within a GIS byshowing a mixture of feature layers. The most sophisticated GIS users are likely to be

working with hundreds of layers, enabling them to create any kind of map display for a

particular geographical location.

Now things start to get more exciting – GIS not only revolutionises the usefulness of map

information, by allowing it to be shown in many different combinations, it also takes us

beyond the realm of the flat, planar view of the landscape. The next section looks at 3-D

mapping using GIS: section 2.7 The third dimension.

2.7 The third dimension

2.7.1 From 2-D to 3-D

To understand a two-dimensional (2-D) representation of the real landscape you need a level

of interpretation and imagination. The physical world exists in three dimensions and, unless

you ignore those extruded plastic maps of the world with snow-capped lumps showing the

main mountain ranges, the realm of conventional maps is uncompromisingly flat. The

capability of GIS to produce dynamic and attractive three-dimensional (3-D) maps is one of

its most exciting benefits.

Map makers use a range of visual symbols to show height information and create the illusion

of an undulating surface:

• Contours

• Spot height symbols

• Hill shading

• Cliff and slope symbols

• Viewpoint symbols



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2.7.2 3-D GIS data

Height information can be captured in a GIS in exactly the same way as the shape and

location of objects. The spectacular ability of today’s computers to perform calculationsmeans that 3-D models of the ground surface can be constructed from data recording the

height at different points across an area.

The typical way this information is stored is an extension of the conventional grid coordinate

system: as well as recording the latitude (the x axis) and longitude (the y axis) for a given

point, the elevation (the z axis – usually as height above sea level in metres) is also stored.

Thus the height information for an area is often referred to in terms of z values. The

fluctuations in ground height across an area are a continuous phenomenon, every point on

the ground has a z value irrespective of whatever physical features are present.

Point height information can be collected by surveyors out in the field, or more commonly byusing remote sensing, including photogrammetry (section 2.3). Points of the same height can

be joined to form a line or contour.

Once created, most 3-D GIS data is stored as a grid of points, with x , y and z values stored

as attributes, often referred to as a digital terrain model (DTM) or digital elevation model

(DEM). From this grid a computer can build a 3-D model.



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An alternative method of representing a surface is to create a triangulated irregular network

(TIN). A TIN model forms a continuous surface by connecting irregularly spaced spot heights

to form triangles, keeping a flat surface within each triangle.

2.7.3 Realistic models

These 3-D models can be made to look very realistic by applying colour to the surfaces. It is

even possible to drape raster images of maps or aerial photos over the surface with quite

stunning effect. Furthermore, if the heights of physical objects like buildings, forests and

electricity pylons are known, these can also be built into the 3-D model. Hence it is possible

to create computer models of entire towns and villages which relate directly to the real world.

The ability of GIS to operate in three dimensions has many useful applications, for example:

• visualisation of the 3-D landscape;

• calculation of gradients for roads and railways;

• environmental impact analysis for engineering projects;

• screening of objects such as power stations and wind turbines through line of sight

analysis;

• radio wave propagation analysis – important to mobile communication networks;

• flood risk analysis;

• town planning; and



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• leisure products – many computer games use realistic landscapes based on GIS height

data.

Below is a selection of thumbnail images showing different types of 3-D view captured from

GIS, have a look at them to see just how effective these visualisations can be. Try

downloading the animations for a glimpse of how GIS can build entire virtual worlds.

Mobil 1 Rally Championship computer game using Ordnance Survey data

Ben Nevis fly-through Snowdon fly-through Scafell fly-through

And now for something completely different. Topology is one of the most revered examples

of jargon in the whole subject of GIS. The next section demystifies this term, which is actually

rather important: section 2.8 Topology. It’s all about relationships.



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2.8 Topology. It is all about relationships

2.8.1 It is all about relationships

While the term topography describes the precise physical location and shape of geographical

objects, the term topology is more concerned with the logical relationships between theposition of those objects. For example, in a topographic map of Hyde Park, London, you

would show an accurate depiction of the shape of the park and a precise alignment of the

shape of the objects within it – Serpentine lake, for instance.

In a topological map the precise shape of the objects is not important – there will be a shape

called Hyde Park and a shape called Serpentine lake, but most importantly the Serpentine

lake object will be entirely contained inside the Hyde Park object.

It is the knowledge of this spatial relationship which is key. This may seem a dry and obscure

point, but topology is critical to understand how the computer is able to analyse the

relationships between objects. If the topology of a set of data is wrong then the GIS cannotanalyse how objects relate to each other: are they next to each other? Do they overlap? Do

they form a connection? Does one lie completely within another?

Geospatial data will have topology inherited from the source material. Hence, when you

digitise a map, the topology, which is implicit in the visual interpretation of the map, is built

into the data. However, care is needed. Unless the data is topologically correct the computer

will not necessarily pick up the relationships.

One of the commonest errors when digitising data occurs when there is a slight inaccuracy in

the start or end point of a line. This can result in the linework not being correctly joined up.

The line can form an undershoot or an overshoot (see diagram below).



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Although these errors can be difficult to detect by the human eye, they prevent the GIS from

understanding the fact that these two features are actually joined to each other.

The two most important aspects of topology in GIS are link-node topology and polygon

topology.

2.8.2 Link-node topology

A GIS does more than just show the positions of objects on a computer screen, it can also be

used to model real-world events. One of the most important examples of this is the ability to

model networks. There are many networks in geographical data, such as water courses and

street maps. A GIS can analyse the potential flow around these networks, a useful ability in

flood analysis or route finding. It can only do this if the data has correct network topology –

the joining of the lines at exactly the same point in the data. Lines in a GIS network are

usually called links, the points which define the shape of the link are called vertices, and the

points at which they join are called nodes.

We will learn later in chapter 4 how link-node topology can be used in network analysis.

2.8.3 Polygon topology

Area features are defined in a GIS by the linear shape of the perimeter and some kind of

reference point indicating that the space enclosed by those lines relates to a geographical

feature. This reference point is referred to as a centroid , a seed or a label point.



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Geospatial data is often captured in the form of linework showing the extent of physical

features on the ground, like fences, roads, and rivers. The area features then have to be

identified by assigning seed points to each bit of space in the resulting map data. For

example, a wall feature may, at one point of its length, define the perimeter of a school

playground, but further along form the edge of someone’s front garden. It is the seeds that

store the information about which links make up the edge of an area feature and what it is. It

is very important to avoid undershoots in the data, otherwise the system cannot tell whether an area is closed at a particular point.

There will be much more about the structuring of data in Chapters 3 and 4.



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3 Adding real-world information

3.1 Introduction

The previous section talked about how the geospatial element of map data can be

incorporated and viewed in GIS. However, to really uncover the power of GIS, we need to

look at what happens to the attribute data.

This information tells you not just the shape of a feature but what it is, and any other possiblepiece of information that may exist about it. A map may show that a feature is a river by

depicting it as a blue line; it may also record its name using a blue text label. In a GIS this

information may be stored within the data itself.

There are numerous possibilities for exploiting this attribute information. Any information that

relates to a place on the ground can be loaded into a GIS and analysed. When you consider

the amount of detail contained in GIS data, this can often raise the question – Is Big Brother

watching you? Well, yes actually, he probably is. We are now well and truly living in an

information age and there is no escaping the fact that some of this information is about

ourselves. The difference with GIS is that if Big Brother is watching us, he is less likely to be

looking at us in the wrong place.

This section looks at the different ways in which the information attached to GIS objects can

be interrogated and exploited.

3.2 The attributes of map features

3.2.1 What are attributes?

The maps shown in GIS are intelligent – the features know their own identity.

How?

In chapters 1 and 2 the ways in which geographical information can be loaded into a

computer and displayed in a GIS have been described. We now move on to show how this

information can be used. The term attribute describes any piece of information about an

object that can be stored in addition to its geographic properties. For instance, a road may

have a number, a name, a maximum width, a speed limit and so on. GIS can work with this

descriptive attribute information to create intelligence way in advance of what can be

achieved by placing text on a paper map.



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With GIS you are no longer restricted by how many text descriptions can be fitted into the

available space to convey information about the objects in an area. Tabular information can

be stored about each of the objects just as in a database, so allowing an almost infinite array

of attributes to be recorded. All GIS have simple tools that allow the interrogation of the

features. Hence, by using your mouse to click on an object, a full set of attributes can be

displayed without that information having to be on screen all the time. The object of interest

can be identified by the visual map graphic and then that object can tell you its ownattributes. Which is very clever stuff.

Move your mouse over this graphic to see the attributes that are associated with the house

and the road.

3.3 The attributes of map features

3.3.1 Attribute tables

Most GIS enable the user to view the data in tabular form without necessarily using map

graphics at all. This is equivalent to using typical office spreadsheet software. Often you may

know the name of an object but not necessarily where it is, hence, you can use the table tofind the object and then switch to the map to see where it is.

• Move your mouse over the table below; notice how it also locates the object on the map.

• Move your mouse over the map below; notice how it finds the object’s attributes in the

table.



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The GIS forms a constant link between the attributes and the geographical properties of each

of the features: you can get either one of these if you know something about the other. This

is the basis of location-finding mapping services on the Internet: you can generate a map for

any location because there is a data layer with a link between the postcode attribute and the

geographical coordinates. You can see how this works on the Ordnance Survey Get-a-map™

pages, but please press the back button to continue with The GIS files.

GIS can be used to link to any piece of information that may exist about an object from other

systems. As we will see in section 3.4 GIS can tell you everything worth knowing about

anything.

3.4 GIS can tell you everything worth knowing about anything

Once a feature is loaded into a GIS, any piece of information about that object can be linked

to it. How does this work?

When you start with geospatial data you often only have attributes that could be determined

from the original source material. Information gleaned from the original map might, for

example, show a line feature as an A road, numbered A11. However, the GIS can be used to

link to any piece of information that may exist about the object from other systems. This can

often lead to very powerful applications of GIS.

Any organisation which holds information about geographical objects can load that

information into a GIS as long as they have some map data containing the relevant objects.

Therefore it is not just the attributes that come with the geographical data that can be

interrogated but any other item of information known about the object.

For this to work it is necessary to have some kind of common referencing system so that the

correct record in the geospatial data can be matched with the corresponding record in thenon-geospatial data.

Use the graphic below to run through this example.

1 This shows you the Ordnance Survey

data about this river and a map

showing its location.



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2 This shows the Ordnance Survey

data and the environmental data –

notice the different information stored

in each table and the common

reference.

3 This shows all the information joined

together and its location on the map

This process has allowed us to link other attributes (environmental information) to the map.

This kind of application is very much dependent on the ability to establish links between the

entities in the two sets of information. Often it is better to use a numerical referencing system

understood by all users of a particular type of information, so that the specific features can be

identified unambiguously. If you just use text names this can fall down if one set of

information has a misspelt name or if there are duplicate entries. There are, for example,

many stretches of river in Britain with the name attribute River Avon.

Ordnance Survey has developed its own common reference system using millions of

Topographic Identifiers (TOIDS). These are unique 16-digit numbers applying to every

feature in its large-scale database. TOIDs will make it a lot easier for users to link, combine

or transfer information quickly and efficiently. This system is part of a massive project known

as the Digital National Framework™ (DNF™). We will cover the DNF in later chapters.

Once the GIS is populated with feature attributes, the layers can be analysed in many

different ways using queries and selections. We will now look at that in section 3.5 UsingGIS? Be selective.

3.5 Using GIS? Be selective

You can query the features in GIS map layers by selecting and viewing just those that satisfy

particular criteria. How useful is that?



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Performing selections on information held in spreadsheets and databases is the classical

way in which computer users make sense of large volumes of data and provide answers to

specific problems. Within GIS it is possible to do just the same, only with the added

advantage that the results of those queries are displayed in a geographical context. So, not

only can you identify which records satisfy a particular set of criteria, you can also see where

they are in relation to each other.

Data stored in tables is usually very difficult to digest all at once. It is necessary to filter out

relevant sets of information corresponding to a particular group of conditions. For example,

imagine you are visiting a city with which you are unfamiliar. You might have a map showing

the city centre and the location of all the restaurants. With GIS range of that fit an

You can make this type of query as simple or as complicated as you like, as long as the data

fields are there to interrogate. This ability is not unique to GIS software; many different types

of information system will allow you to perform selections. However, only GIS can provide a

visual representation of the location of the query results. Furthermore, GIS can apply

geographical criteria to the selection filter such that objects are selected based on where

they are. We will cover this in more detail in Chapter 4.

The next section, 3.6 Geocoding builds on the idea of integrating tabular data by explaining

the term geocoding, another favourite example of GIS jargon.

3.6 Geocoding

This is one of the key functions of GIS, but what does the term geocoding actually mean?

In a way, the concept of geocoding is very similar to the idea of linking to external datasets,for which the river data example was used in section 3.4. Geocoding describes another way

of importing non-map data into the GIS such that its geographic properties can be identifiedand the records positioned in space. However, unlike the linking method in which theadditional attribute table remains external to the mapped layer, when a table is geocoded itbecomes a new map layer in its own right. Coordinate points are assigned to the geocodedtable so that it can be used on its own to display the locations of the objects concerned.

OK, we may have lost you, so… Geocoding is easier to explain with a worked example:

It usually takes place with a list of locations with known addresses. Imagine you have asimple table of British football clubs containing the name of the club and the postcode. Togeocode this list you have to process each record against the postcode data in the GIS.There are already several GIS data products that store the National Grid coordinates for every postcode in the country. The National Grid coordinates from the postcode product getcopied across to join the football club list. This creates a new football club layer that can beadded to the GIS.



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Geocoding is often applied to address lists. Not many people know the National Gridcoordinates of where they live so any list of people’s addresses needs to be geocoded toload it into a GIS. Any company, a bank for example, which holds an address list of customers can geocode this information and instantly analyse their geographic distribution.This may reveal trends that the bank would otherwise be unaware of, areas of particularlyhigh or low customer density, which may suggest reasons for the successful recruitment of customers. We will look at how organisations are using GIS to improve their decision makingin greater detail in the next chapter.

Finally, in the last section of this chapter, we look at the way that geospatial data isstructured and can affect the amount of information which can be analysed in a GIS:section 3.6 Structured GIS data is the key .

3.7 Structured GIS data is the key

3.7.1 The significance of structure

For geospatial data to be really useful to GIS applications it must be structured to model thereal world. What is the significance of structured data?

In the last few pages we have seen that there are a number of ways of getting general typesof information about objects into a GIS. This always relies on the presence of a layer storingthe location of a set of objects along with some attribute like a name or an ID number.

Our examples so far show that:

• to link environmental information about rivers we needed a river map layer possessing

names as attributes; and

• to geocode a set of addresses we needed a geographic layer with the coordinates of postcode locations together with the postcode text itself.

In these examples the GIS must hold location and attribute data that corresponds to thephysical objects which people want to analyse. This means that the structure of the physicalobject data in the GIS data must be attuned to the types of application that the system ismeant for.

Do not worry, this will all become clearer as we move through this chapter and the next. Theidea is that data has to be structured in a certain way to carry out certain types of spatialanalysis.

We will now look at Ordnance Survey point data that is often used for geocoding andOrdnance Survey road network data that is often used for analysis of networks.



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3.7.2 Address data

Ordnance Survey produces data products that have their origins in the accurate large-scalearchive but which are tailored for use in GIS. The simplest is ADDRESS-POINT®, wherebythe building seeds from Land-Line® have been matched with the Royal Mail Postal AddressFile (the UK authority on addresses). This has created a basic point dataset where the

location is stored as a pair of National Grid coordinates along with the address attributes, likehouse number, street name and postcode. This data is as far removed from a conventionalpaper map as can be imagined.

On its own, ADDRESS-POINT has no capacity for visual interpretation as a map. It is purelya resource to enable address-based information sources to be loaded into a GIS for analysis

3.7.3 Road network data

One of the major uses of GIS is in the modelling of transport networks. To get a reallycomprehensive model of a road network we have to look to large-scale sources for the mostaccurate possible information. However, the data attributes that are captured in the sourcemapping are the physical objects such as the roadside kerbs and the fences bordering thepavement. You can see that it is a road but there is nothing necessarily in the data to definediscrete chunks of the road network. This is why many mapping agencies have capturedroad centrelines in addition to the physical features shown on paper plans, as shown in thegraphic below.



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Ordnance Survey has generated a set of road centrelines showing the complete roadnetwork from the accurate information in the large-scale data, the resulting family of productsis referred to as OSCAR® (Ordnance Survey Centre Alignment of Roads) data. Again, inisolation this data does not appear helpful in a visual sense. However, with its link and nodetopology (see Chapter 2) and attributes about road names and numbers, OSCAR is the basisof many powerful GIS applications.

1 OSCAR data

2) OSCAR displayed over 1:10 000 data backdrop

3 OSCAR data using colour to show traffic volumes

The third graphic shows how traffic volumes may vary on a road network: the red roads carry

the highest volume and the green the lowest. The ability to analyse a network like this is

useful for finding problem areas and suggesting alternative routes.



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3.7.4 Spaghetti data

Large-scale topographic data is normally captured by the survey of physical objects on the

ground. Before the advent of GIS the need for structured data was not understood, so data

was captured quite randomly. Often a line along a connected series of objects was captured

as a single feature, instead of a separate line for each object or house. This is especially so

in housing estates, where long fences or walls surrounding a series of properties weredigitised in one go. When the data is drawn on screen it looks fine; you can see a row of

houses, each with its own front and back garden. However, a look at the attribute table

shows that the data does not consistently correspond to discrete real-world objects like the

wall of a single house or the house itself.

This kind of data is often referred to as spaghetti data (as unstructured and random as

spaghetti thrown onto a plate). The lines are a tangled mixture that can be interpreted

visually as a large-scale map, but they do not explicitly store each separate datum that the

GIS could potentially analyse. Put simply, even though it looks good, it does not make much

sense and it is not very useful for analysis. There is much more need to analyse informationabout houses, properties and land parcels than there is about the linear features which

enshrine them; only a creosote manufacturer is likely to be interested in analysing the

attributes of fences.

Now lets look at how improving this data structure provides significant benefits to GIS users.

3.7.5 Polygon structured data

It is possible to create large-scale data that enables the identification of every single discreteparcel of land from spaghetti data. Many different GIS programs are able to automatically

convert spaghetti data to polygon structured data by identifying every bit of space betweenlinework. This means that the data is much more useful as it represents the real world in amuch more realistic way. Again the data looks good, but now it makes much more sense andcan be used easily straight off the peg for complex analysis.



The GIS files


4 Putting it all together as a system

4.1 Introduction

With the different features described in the previous chapters, it is clear that GIS are veryflexible and sophisticated tools. Not only can they understand the location element of mapdata and manipulate information about shapes and structures, they can also work withattribute information to store intelligence about objects. It is the fusion of these two functionsthat makes this tool so powerful.

This chapter looks at the different ways in which GIS can provide practical solutions.

4.2 Unlocking the information

All organisations have information locked away in various databases – GIS can help uncover the full value of this information.

Approximately 80% of all information held in databases anywhere in the world contains somekind of geographic element. For example, records in a database can be tied to a particular location on the ground, such as an address, building, property or road junction. There are

many trends and relationships hidden in this geographic data, but it is only by using a GISthat these are revealed.

Many different organisations use GIS as a central part of their activities, and the range of applications in use is extraordinary.

For example:

• utilities – leak management, service planning, network planning;

• central government – census, environmental planning, health service catchment areas;

• local government – refuse collection, street lighting, council tax collection;

• emergency services – crime locations, route finding;

• military – battlefield simulations;

• retail – travel time catchment areas, store site location;

• financial – insurance flood risk, property values; and

• target marketing – demographic profiles.

One of the best ways to analyse data is to produce colour-coded maps that reveal patterns indata which may otherwise be missed: this is explained in section 4.3 GIS reveals all .



The GIS files


4.3 GIS reveals all

4.3.1 The thematic map

By bringing together data from a wide range of different sources, you can visualise trends inthe data by creating thematic maps. How does this work?

We discussed in previous chapters how a GIS display of map information is very flexible.Unlike a paper map it does not require every piece of information to be visible at the sametime. It can also change the depiction of a particular object depending on the value of one of its attributes. This function is known as thematic mapping. You will already be familiar withthematic maps from atlases and geography textbooks. For example, a map of parliamentaryconstituencies shaded in different colours can show the number of seats held by differentpolitical parties. GIS can build this kind of map automatically from the data values (number of seats), and in many different ways.

Thematic maps come in all shapes and sizes, for example:

• a map of farmer’s fields showing a

different colour for each type of crop

grown;

• different sized point symbols to show therelative population of towns;

• a display of a road network with differentcolours to show average traffic speeds;

and



The GIS files


• a density map to show average numbers

of badgers across different counties.

These examples are typical of the types of thematic mapping for which GIS is used. Thereally dynamic thing about these maps is that they can automatically change their appearance as the values in the data tables change with time. Hence, you can use this kindof mapping to constantly monitor traffic flow.

4.3.2 Visual analysis

The graphic below shows a range of different thematic map layers. There are basically twodifferent layers here. The first layer displays the relative turnover of a set of burger barsacross a city (the blue dots) and the second layer is background mapping. This second layer can be switched between a series of data layers showing a different set of attributeinformation for the same area. Try swapping the layers around and see if you can spot acorrelation between the background layer and the burger bar layer. Which layer shows apattern that fits with the more successful burger bars? Why might these layers be related?

Have a look…



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This example shows how GIS can be used to visually analyse information and help to explainspatial patterns.

However, you do not have to rely on a visual interpretation of trends in data - the GIS can dothis for you. This is explained in section 4.3 What happens where? – The power of the spatial query .

4.4 What happens where? – The power of the spatial query

4.4.1 Basic spatial querying

GIS can not only tell you what information exists about particular features in the map data butit can also analyse where things are in relation to each other. What can this tell us?

GIS can go beyond visual analysis of thematic mapping as described in the previous section.The software can identify trends across a given area as well as performing specific queries.

Such queries can select attribute data depending on its geographical location and theninterrogate the attributes by performing calculations and statistical analysis. Selecting databased on the geometry of objects is known as performing a spatial query.

The simplest spatial query can be performed on screen using the selection tools that areprovided with the GIS software. For instance, you can draw a circle on screen and select allobjects falling inside it. This example shows addresses that have been selected becausethey fall within the circle.



The GIS files


This technique could be used by the emergency services to quickly identify all houses within500 m of a spillage of a dangerous chemical.

4.4.2 Buffers

It is possible to analyse how close objects are to one another using a buffer. A buffer is a

shape based on any other existing object (point, line or area) that can be generated by theGIS. The buffer object represents the total area within a certain distance of a given feature.

Example of a point buffer

Example of a line buffer

Example of area buffer

You can use the GIS to generate buffer zones and then identify all features that lie within aparticular distance. For instance, you can select all addresses within a 500 m buffer of a busyroad and compare these with data about the incidence of asthma. By comparing both sets of data you can work out if there are statistically more asthma sufferers living in the buffer region than in the general population. This allows you to analyse whether proximity to a busyroad is likely to be a factor in the cause of asthma.

4.4.3 Overlay operations

As well as drawing simple shapes and calculating buffers to select objects, it is possible toplace layers on top of each other (remember we looked at combining layers in Chapter 2,section 2.6 Looking at multiple layers) and select all objects from one layer that lie within anobject from another layer.



The GIS files


4.4.4 The tricky but important bit...

A key advantage of being able to layer data in a GIS is to carry out overlay operations. Thesecan be quite complex, but simply mean combining layers of data to create one new layer (similar to the way a mathematical sum or calculation creates a new value or answer).

In the example below a farmer needs a certain level of rainfall and a type of soil tosuccessfully grow a crop. By combining the rainfall map and the soil type map it is mucheasier to find the best location. In this example the GIS assigns a numeric value to each soiltype and to the amount of rainfall. This makes it easier, on the resulting map, to see wherethe optimum growing area is located.

The point is that by combining layers of information, the farmer creates a new map that ismuch more useful.

Overlay operations are particularly effective when using raster data. As discussed in

Chapter 1, raster data is good for representing the continuous varied surface of the earth,whereas vector data makes assumptions that the edge of an area has a defined boundary.An example of a vector overlay can be seen in the graphic above. The soil type areas areclearly defined, whereas in reality we know where two soil types meet they often graduallyblend into each other. Another good example is using aerial photography to analysevegetation. It would be very difficult to draw onto a photograph where one area of vegetationstopped and another began: the vegetated areas would be blurred into each other andprobably produce a speckled effect in the photograph.

GIS users must never forget that the result that a GIS provides is only as accurate as thedata that was used for the query. Do not forget that rubbish in equals rubbish out.



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Spatial querying is used in many different ways to help understand the world around us.Often the answer to a particular problem can only be unravelled by comparing two layers of information in a way that would be almost impossible to achieve without the GIS software.The next page looks at a much more specific GIS application that is increasingly becoming apart of everyday life.

One of the most dramatic successes of the GIS revolution has been the development of intelligent navigation systems using digital models of road networks: section 4.5 Show methe way to go.

4.5 Show me the way to go

4.5.1 Network analysis

One of the most far-reaching applications of GIS is in network analysis. Network analysis isthe mathematical processing of the geometry of a link/node layer, enabling the identificationof all possible routes around that network, along with the distances and times involved. Put

simply, this means that, using an accurate road data layer, the computer can identifypossible routes between two locations and calculate the shortest.

Okay, now we have definitions out of the way, let us remember what we covered inChapter 2, section 2.8. Way back then, we discussed the concept of link-node topology andhow important it is to have the data structured correctly (that is links joining at nodes with nogaps). In order to carry out network analysis you need a link-node data layer of line featuresrepresenting a real-world network (for example, a road network); only then is it possible tomodel movement around that network.

The simplest example of network analysis is to choose two points on the network and ask the

GIS to calculate the shortest path between them.

This basic concept can be used to help build navigation systems and to plan distribution

services.



The GIS files


4.5.2 In-car navigation

By applying the principles of network analysis to accurate road data, you can build systemsfor motorists to navigate. Many cars now have gadgets that provide driving instructions,either as a simple map display or by audio messages. These in-car-navigation tools areactually specialised, miniature GIS. Because the data has attributes for road names and

numbers, intelligent instructions can be provided (for example, ‘take left turn A34 at next junction’).

In-car navigation requires up-to-date map data and extrainformation to make the data model behave like the real world:you need to know which roads are one-way streets or wherethere are no-right-turn signs. By using unique identifiers in theroad data so that each link in the network can be pinpointed,additional information can be built into the system. Furthermore, itis possible to receive real-time information about traffic conditionsas you drive, so that you get advance warning if there arehold-ups due to road works or an accident.

4.5.3 Drive-time analysis

Another benefit of network analysis is the ability to calculate drive times, which identify howfar you can travel in a certain amount of time. The typical drive-time map, for example, for pizza delivery, would show a central point surrounded by a series of circles estimating howlong it takes to get to places within that radius. This method assumes an as-the-crow-flies

route to each location.

A GIS can be much more accurate – it can use network analysis to generate isochrones (lines that join up points of equal travel time) that take into account the true road network andgive a proper measure of how far you can get over a set time. This can even take intoaccount the average speed on each road, so that the area appears stretched along faster roads.



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Many different organisations use this kind of drive-time analysis to plan their operations, fromthe siting of new stores to the planning of distribution networks.

4.5.4 Optimum-path analysis

Network analysis does not have to be carried out on vector link and node data. As discussedin the previous section, Overlay operations, raster data can be effective in describingcontinuous varied surfaces. This quality of raster data is useful for identifying the optimum path (path of least resistance or shortest path) through a continuous surface. For example, acompany needs to erect electricity pylons from A to B.

They need to make sure that they disrupt the forest areas as little as possible. The GIScalculates the path of least resistance, by finding the path that adds up to the lowest value. Avector line can then be added to show the location of the proposed route.

Finally in this chapter we will look at some examples of how different types of organisationbenefit from the range of functions described in the previous pages: section 4.6 Some simpleGIS examples.

4.6 Some simple GIS examples

4.6.1 How many useful applications can GIS provide?

We have now looked at a wide range of GIS concepts. GIS can be used to speed up anyprocess that formerly relied upon using paper maps. The analytical functions of GIS meanthat geographical information can be used in unprecedented ways.

It is important to appreciate that GIS does not always provide exact answers to problems, butby identifying trends based on geography, GIS can reveal patterns that can help us makeinformed decisions. A GIS can improve decision-making; it cannot make decisions for us.

On the next few pages are some typical applications to complete the GIS picture. They showhow GIS is helping to improve everyday life.



The GIS files


4.6.2 Flood risk

Using 3-D height data and map data for river features it is possible to build a computer modelof changing water levels; this can be used for predicting flood patterns and identifying areasin danger. By combining this model with address data, the likelihood of individual propertiesbeing flooded can be assessed. This is not just of environmental concern but of great value

to insurance companies.

4.6.3 Emergency services

By using the GIS as a computerised map, controllers of police vehicles and ambulances caninstantly call up a detailed map of the area around an incident. By tracking the vehicles inreal time and using route-finding GIS functions, the controller can identify the best vehicle toattend and give directions for the fastest way to the incident. They can even store historicalinformation and look for incident patterns and black spots.

4.6.4 Estate agents

A GIS makes an excellent system for providing information to potential house buyers aboutthe houses for sale in a particular area. By allowing selection based on price, number of rooms, type of house and so on the display can instantly show the range of properties fittingthe requirements of the customer (similar to when we selected restaurants in Chapter 3,section 3.4). The system can then go on to provide information about the local amenitiessuch as schools, shops and recreation facilities. Several of these systems are alreadyavailable on the Internet; examples are below:

• http://www.propertybroker.co.uk

•

http://www.national-property-register.co.uk



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Go to search for a property:

• http://www.home-envirosearch.co.uk

4.6.5 Nature conservation

GIS can be used to record locations as part of a nature conservation project. The value of the GIS is in providing instant access to information about the geographical spread of sightings or plantings so patterns can be detected. It also allows for a user-friendly way for individuals to input information into the system.

An example is below:

• http://www.mammalstrustuk.org

4.6.6 Retail

Supermarket chains use GIS to help site new stores and to plan their distribution networks.

By comparing how many people live within 15-minutes drive time of a particular location withthe number of supermarkets already trading in that area, the GIS can identify suitablelocations with an optimised catchment area. Supermarket chains also use socio-economicdata to create profiles of the people in their catchment areas to help them understand whichother parts of the country are likely to be successful growth areas.

4.6.7 3-D environmental impact analysis

By building a 3-D model of a landscape it is possible to simulate the construction of a newfeature which may have an impact on the natural beauty of an area. For example, planning awind farm. By using accurate map data for the area, a realistic model can be created andviewed from all angles. This will help identify the location that the new wind farm will have theleast impact upon.



The GIS files


4.6.8 Airport-noise pollution

Restrictions on the permissible levels of aircraft noise affect all busy airports. GIS can helpmonitor not only the noise itself but also complaints from nearby residents. The spread of sound from the airport can be mapped against the nearby built-up areas to identify how many

houses are going to be affected by high noise levels. By logging the addresses of peoplewho complain about noise, the airport can monitor the effectiveness of their noise controlmeasures and whether or not the airlines are obeying guidelines.



The GIS files


5 Case studies

Chapter 5 refers to other parts of the web site rather than being text in the GIS files. See

http://www.ordnancesurvey.co.uk/business/studies/index.htm for more details.

http://www.ordnancesurvey.co.uk/business/studies/index.htm





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6 Chapter 6 – Expert GIS concepts

6.1 Introduction

From the embryonic early days of computerised maps, GIS has now grown into a fully-

fledged science in its own right. Different aspects of geography and computing combine to

create a complex and dynamic subject area. GIS as a discipline can now be followed in

higher education and as a professional career. There is a wealth of literature written on the

subject, in journals, textbooks and on the web.

These pages do not attempt to provide a comprehensive, in-depth description of every

aspect of GIS. But in Chapter 6 Expert GIS concepts, we detail some of the more interesting

and current GIS issues. Hopefully, this will emphasise just how far reaching the subject has

become. In Chapter 6 Expert GIS concepts, you will discover the importance of data formats

and standards, the impact of database technology on GIS evolution and how GIS hasadapted to the Internet.

This chapter will also show how a diverse range of new types of map can be generated by

GIS software and, in the section on location-based Services (LBS), why many predict an

explosion in the use of geographic data in everyday life through the medium of handheld

computers and mobile phones.

So read on to learn about a selection of more advanced GIS concepts.

6.2 Data formats

6.2.1 GI data compared to other data

In its most elemental form digital data is composed of bits: indicators that have a state of

either 0 or 1. Information can be encoded in these binary characters. The way in which this

code works varies between systems.

One of the most universal conventions is the organisation into sets of eight bits, called words

or bytes. A byte therefore has a sequence of 0s and 1s in any of 256 combinations, be it

00000000, 11111111 or 01101000. These are essentially the same as the set of numbers in

base 2 equating to the decimals 0 to 255. Streams of bytes can be used to encode all kinds

of information, the power of the computer comes from the volume in which these streams can

be stored, and the speed with which they can be transmitted and manipulated.



The GIS files


The file is a collection of bytes that make up a logical unit of information. In describing types

of file, the terms ASCII and binary are commonly encountered. ASCII (American Standard

Code for Information Interchange) files adopt a convention in which each eight-bit sequence

corresponds to one of a set of common characters. ASCII files are very simple and can be

created using basic text editing tools like Notepad. A binary file encodes information in the bit

sequence. You can only interpret the information contained in it by knowing the code for that

particular file. If a file is described as binary it means that information is encoded in the bitsequence in some way or another – only by knowing the code for that particular file can you

interpret the information, be it text, graphics, mathematical formulæ, video or whatever. This

is how different software products use different types of file, identified with their different file

extensions (for example, .doc, .tiff, .java and so on). In a sense, all files are binary, but in

common usage the term refers to a file that does not conform to the ASCII convention. Try

this useful link for more information about ASCII codes.

The data used in GIS is no different. It is also organised into files with different software

products using their own particular file types, binary coding and file extensions. The earlier

chapters of The GIS files described how geographical data comes in many different forms.

This fact is reflected in the files that are used to store this data. The range of different filetypes used in GIS can be very confusing! Word processing packages have a simple use of

file types. With Microsoft®

Word you store each document in a single .doc file. GIS can be

much more complex. With geometry, attributes, indexes, topology, image and history

information to store, most systems use more than one single file (with multiple file types) to

encode a particular data layer. Chapter 6.1 attempts to put these file formats into

perspective.

6.2.2 Proprietary file types

Every software product is designed to work with a specific set of file types. In essence that is

what the software does: it reads the particular binary code to extract the stored information

and then does something with it, for example, displays it on screen, sends it to a printer or

performs calculations. Commercial software products usually have their own specific binary

formats.

It would be impossible to describe the full range of different file types used in GIS as there

are so many. However, it is important to recognise that each product handles the storage of

information in different ways and, to fully understand what is happening to the data, it is

useful to consider the files involved for your own system, what is going on under the bonnet,

so to speak.

http://www.asciitable.com/





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Here are a few examples:

• single file for each layer – in some systems all information for a given layer is stored in a

single file (for example, .dxf files in AutoCAD®);

• multiple files for each layer – some systems use a series of files for each layer (for

example, MapInfo®

has a .tab file for each table of information, but this is just a pointer to

a set of files containing the geometry, attributes, identifiers and indexes separately); and• a folder of files for each layer – more complex systems can use a more complicated

hierarchy of files held in a specific folder to store the information (for example, ArcInfo®

coverages).

In these last two examples you only ever interact with the data through the GIS software

interface. The individual files are not designed to be edited outside the GIS as this will almost

certainly corrupt the data.

GIS can read image data from standard graphics file formats but often need an additional fileto register the image in space. Examples of such files are MapInfo .tab and ESRI

® .tfw .

These are in fact simple ASCII files. If you have any examples on your own computer try

viewing the contents in a text editor; this can be useful to understand how they work. On the

next page we’ll see how ASCII files can be important in the transfer of data between

systems.

6.2.3 Translators and transfer formats

GIS software is designed to work with data stored in specific proprietary binary data formats.

The skill of the software developer is to optimise the system’s performance to include as

many functions as possible while remaining fast and robust.

The binary code used to store the data is critical to the performance of the software and GIS

vendors guard their binary formats as part of their unique intellectual property.



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This means, however, that many different file formats exist. To get a feel for the complexity of

file formats, have a look at these file extension source pages. With so many different

software products available, each with their own data formats, users of one system may,

therefore, find it difficult to swap their information with users of another. In the early days of

GIS this was serious; if you used data in one package you could not use the same data in a

separate system from a different vendor. More recently it has become standard for GIS

software to have import facilities that can open files from a diverse range of formats and storethem in the preferred local format.

There has also been an explosion in the development of translator software. Products have

been developed to convert geographical data between a whole array of formats. It would be

unfair in these pages to highlight a specific translator product in comparison to any other. But

it is now possible to convert between practically any of the possible data formats, of which

there are over a hundred, in either direction. Type GIS translators into a search engine and

see the results.

Most of these formats are binary as in this form data is more closely integrated with the

software engineering of the products and can be manipulated more quickly. Codes to work

with such data can only be written if you know the binary format. There is a series of more

simple ASCII file formats that have been developed to enable easier transfer between

systems. The human readable nature of ASCII files means that it is an order of magnitude

more straightforward for other developers to write programs that can read these files. The

MapInfo MID/MIF format is an example of an ASCII transfer format.

Ordnance Survey has traditionally supplied its vector data in ASCII formats with a relatively

simple, documented structure that can easily be understood by developers. Until recently themain formats used have been DXF

™and NTF. NTF is also British Standard BS7567, used for

the transfer of geographic data, administered by the British Standards Institution (BSI).

http://search.sprinks.about.com/library/dist/sperch3.htm?site=filext&pid=1

http://search.sprinks.about.com/library/dist/sperch3.htm?site=filext&pid=1



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6.2.4 Open formats

The previous pages in this section have discussed why so many different file types are used

in today’s GIS applications. Traditionally, data providers have supplied data in open ASCII

formats with systems simultaneously loading and translating it into the proprietary binary

format. Data can be exchanged between systems where an import option exists for the

particular formats. There are also bespoke translation tools available to cover every possibleoption. Exchange between formats has advanced further in recent years and the term

interoperability has become important. Within a single organisation there can be several

different software products being used and it is imperative that information can be shared

between them.

According to Moore’s Law, the processing power of computer hardware can be expected to

continue improving. It is therefore becoming less critical to the performance of GIS software

for the data to be stored in the optimum format for that particular system. The recent trend for

systems to use non-specific data formats means that data is read from, and written to,

different native formats on the fly as the software performs its functions.

As a data provider, Ordnance Survey has to make careful decisions about the formats it uses

to supply data. Data users want a choice of formats to avoid the need for translation, and

although it is difficult to provide every possible format, excluding just one would be unfair to

that software vendor. This explains the need for standard open data exchange formats that

create a level playing field for the producers of software and translators. The standards being

developed by the Open GIS Consortium are becoming a favoured option, see the XML and

GML page in the next section.

The increasing significance of databases and the Internet is also playing a big role in the

advance of interoperability. Increasingly, systems are being built around the use of databases to hold the information in each GIS layer, replacing the use of flat files. Proprietary

binary formats are therefore becoming less important. A similar effect is seen in the way that

systems can now read data in real time from central locations on a network, rather than

reading from locally stored files. Although this section on data formats set out to highlight

differences between file types, these issues will probably have a greater resonance for those

practicing GIS in the late 1990s rather than today. There is more to come on spatial

databases and the web in forthcoming sections of Chapter 6.

http://www.webopedia.com/TERM/M/Moores_Law.html





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6.3 Standards

Standards are a fundamental part of modern society; an organised way for ensuring best

practice, common design, safety and many other benefits across every field of industry and

science. A series of bodies exist to coordinate and promote the generation of standards.

These bodies now play a crucial role in geographic information (GI) science.

ISO, the International Organisation for Standardisation, is very active in the field of standards

for GI. The ISO Technical Committee – TC 211 – is in the process of producing standards for

many aspects of spatial data, including metadata; spatial referencing by coordinates; imagery

and gridded data; and data quality. For more details of ISO, and especially ISO/TC 211, see

the TC 211 official web site.

BSI, the British Standards Institution, is the oldest national standards setting body in the

world, including BS 7567 (aka NTF) and BS 7666 (Spatial data-sets for geographical

referencing) – see the Gazetteers page. Ordnance Survey is involved with the BSI’s technical

committee – IST/36 – which is responsible for the UK participation in the area of GI in

international committees. Click here f or more details of IST/36.

More recently, OGC, the Open GIS Consortium, has been established. OGC is an expanding

organisation dedicated to the creation of standards in the field of interoperable geospatial

systems. The membership includes most of the GIS and database vendors, several major

spatial data users and a few spatial data producers. OGC develops interface specifications

for geospatial data and systems, and is increasingly involved in prototyping services for

serving and accessing spatial data over the Internet. One of the key areas of activity is

encouraging the adoption of standard formats for geographic data exchange based on

eXtensible Markup Language (XML), see the XML and GML page. The OGC public web site

contains a wealth of information about these activities.

6.3.1 Metadata

Metadata is a word that frequently crops up when discussing GIS. It can be described as

data about data. It can be very useful if files of digital information include additional

information describing the contents of the main part of the file. This concept is inherent in

many commonly encountered file types. For example, many graphics file formats like .GIF or

.JPG contain a header component that does not specify the image itself but describes the

palette of colours present in the image. Similarly, many web pages carry metadata contained

in meta-tags at the top of the file.

http://www.isotc211.org/


http://www.gistandards.org.uk/


http://www.opengis.org/







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Metadata is very important in GIS because many different datasets exist, and it is essential to

know certain things such as who created it and when, the types of feature it contains, the

geographical bounding area and the precision, accuracy and scale. Many standard GIS file

types have a header or separate metadata file as part of the data format, or supply the

metadata as a written report. A good analogy is to liken metadata to the nutritional

information displayed on food packaging.

There has been much effort over recent years in the GI community to create metadata

reference archives so that the full range of available datasets can be identified and made

accessible. By standardising the way in which metadata is stored it is possible to identify

resources that contain common types of information. Therefore, if you are interested in

forestry you can access a metadata gateway and find references to forestry information

stored across the globe.

Try these links: askGiraffe.org.uk (link is now http://www.gigateway.org.uk/US Federal Geographic Data Committee

6.3.2 Gazetteers

At its simplest level a gazetteer is a dictionary of geographical names. Every record contains

a description of the location, providing the user with a simple means of identification and

reference. An electronic gazetteer works in exactly the same way. It is a file or database

listing every feature of a particular type (such as a building, a road or a pond) within a

defined area. The user can locate the position of the feature and query any additional

information attached to it.

http://www.gigateway.org.uk/default.asp


http://clearinghouse1.fgdc.gov/FGDCgateway.html






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6.3.3 Gazetteers in England and Wales

Recent developments in the UK mean that a definitive national gazetteer of certain key

geographical features is closer to becoming a reality. Reliable and consistent gazetteers are

vital for different parties to be able to refer to and locate a feature easily and without

ambiguity.

The first step towards this goal was the New Roads & Street Works Act (1991), whichrequired utilities and contractors to notify local authorities of road works. It also specified that

local authorities should jointly maintain a National Street Works Register .

More significantly, in 1993 the first draft of the British Standard BS7666, Spatial data-sets for

geographical referencing, was published.

BS7666 currently contains four parts:

1 Specification for a Street Gazetteer – an up-to-date list of all streets in an administrative

area. Every entry is allocated a Unique Street Reference Number or USRN.

2 Specification for a land and property gazetteer (LPG) – an up-to-date list of all land and

property units in an administrative area. Each record is called a Basic Land and PropertyUnit (BLPU) and holds data relating to its provenance. A BLPU also holds a grid

reference locating its central point and a Unique Property Reference Number (UPRN).

3 Specification for addresses

4 Specification of a data-set for recording public rights of way

Useful links: The National Land Information Service The National Street Gazetteer The National Land & Property Gazetteer

6.3.4 XML and GML

In general, mark-up languages use tags to associate a rule to interpret the content of a set of

information. In HTML this means the visual formatting and association of HTTP hyperlinks

with text and images. For example, the <font> tag can be used to instruct a browser

application to display a piece of text in a certain style.

http://www.nlis.org.uk/


http://www.nsg.org.uk/


http://www.nlpg.org.uk/







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In a similar way, eXtensible Markup Language (XML) uses tags to give meaning and context

on the content of a set of information. In an XML document, tags could encode the fact that

Southampton is a city. But alternatively they could state that, in a particular document,

Southampton refers to a node on a network of shipping lines, or in another, a football club.

This is where the extensible bit comes in. In XML you can define your own set of tag types as

long as the tag set applicable to your document is defined in a separate schema. XML

provides for self-describing data. This makes it very useful as a standard format for exchanging information because computer programs can interpret the content of XML

packets without any prior exposure. XML makes system development more flexible and is

rapidly becoming the standard for information interchange on the Internet. For an introduction

to XML, visit www.xml.com/pub/a/98/10/guide0.html

The emergence of XML has led to the creation of a wide range of mark-up languages

specific to particular subject matter such as maths, chemistry and medicine. There is also a

Geography Mark-up Language (GML) that allows spatial data to be stored and transferred

between systems over a network. It allows points, lines and polygons to be encoded along

with their attributes and the spatial reference system on which they are based, for example,

the National Grid. There is already a lot of interest in GML from other communities, includingthe mobile phone industry and the general Internet community. GML is fast becoming the

definitive method of describing geographical data and simple location information on theInternet. The specification for GML 2.0 can be viewed on the OGC public web site.

Ordnance Survey has adopted GML as the format for OS MasterMap™.

6.3.5 Future standards

Standards are continually being developed and enhanced, and GML is no exception. Many

leading software suppliers, data providers and GIS users are involved in coordinating the

way the GML standard is evolving. There are other OpenGIS®

standards currently under

development that will also play an important role in the next few years.

OpenLS is an initiative for standards in the location-based services (LBS) arena (see

section 6.7 ). OpenLS is designed to make geospatial data and services more widely

accessible through PDAs and mobile phones. According to OGC, the vision of OpenLS is ‘to

deliver open interfaces that enable interoperability and make possible delivery of actionable,

multi-purpose, distributed, value-added location application services and content to a wide

variety of service points, wherever they might be, on any device’.

http://www.xml.com/pub/a/98/10/guide0.html


http://www.opengis.net/gml/01-029/GML2.html


http://www.ordnancesurvey.co.uk/osmastermap








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OGC have set standards for web mapping that are now being developed in the area of web

services (see section 6.6). The OWS-1 initiative takes the results of previous web mapping

testbeds and develops them further, with the aim of ‘developing interfaces to enhance the

growth of geospatial web services’.

Standards in the wider mobile phone market are also going to be important in the way LBS

evolves. Recent years have seen the phenomenal, and unexpected, success of the shortmessage service (SMS) for sending text via mobile phone. With the promise of much faster

transfer speeds from GPRS and 3G networks, combined with colour screen smartphones

with embedded digital cameras, the standard for exchange of multimedia messages (MMS)

will be important, especially as this will be the method through which GI will be visualised on

these devices.

The Japanese are a particularly useful example for demonstrating the importance of

standards. Standards are established very quickly in Japan through cooperation between thestandard’s bodies and industry players. This enables new technology industries to be

developed very quickly. The success of the i-mode system in Japan demonstrates this. Since

starting in 1999, this colour screen, Internet access, mobile phone system has grown so

much that by March 2002 i-mode claimed 31.3 million subscribers (25% of the population)

and 53 000 compatible web sites. This is a stunning example of the benefits of standards

and, as you can see, this will be a crucial issue for GIS in the future.

6.4 Spatial databases

6.4.1 Database fundamentals

Section 6.1 Data formats describes different types of data file. Files are the most commonly

used packets of information in the world of the desktop computer. But when the volume of

data becomes very large, or you need to allow many people to access the data at the same

time, it becomes preferable to store the information in a database.



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A database is a tool capable of storing large amounts of complex information in a structured

way. Information in a database is organised into individual records that can be referenced,

sorted, indexed, linked and queried. Most computer systems you interact with on a daily

basis have some kind of database behind the scenes. These contain many different types of

information, for example, an ATM showing your bank account details; on-screen flight

information at an airport; products and prices at the supermarket checkout and so on.

Databases are, in principle, more robust, secure and scalable than storing information in flatfiles.

Database technology is a very large subject and can only be covered in very simplistic terms

in these pages. In large industrial software systems there will usually be multiple databases

operating together in a database management system (DBMS). There are two distinct major

database types: relational (RDBMS) and object-oriented (OODBMS). This section

summarises the difference between these database types and explains how database

technology plays a big part in GIS.

6.4.2 Relational databases

The storage of information in a relational database is fairly simple. The records of information

are organised into rows and columns in a table, with a separate row for each entity and a

column for each property stored about that set of entities. For example, a table could contain

a list of cars with each entry containing values such as a registration number, colour,

mileage, make or mode. This is essentially the same as a spreadsheet. Each column is

formatted to store a particular type of data, be that text, numbers, dates or boolean. Thesetables can be queried just like the earlier examples of selecting data in a GIS from

section 3.5 Using GIS? Be selective. A structured query language (SQL) that is used by

almost all RDBMS to allow interrogation of the data has been developed. For example, the

SQL statement Select * from CARS where MAKE = ‘Ford’ will generate a list of all the Fords

in the original table.



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For the computer to understand the data correctly there can be no duplicate records in a

table. In a relational database each record in a table must be uniquely identified for queries

to be meaningful. This usually means that one of the columns should contain unique values –

the primary key . In our example you could assume that car registration numbers will never be

duplicated so this column could be used as the primary key. Often the data does not

inherently contain a primary key, so the database will generate its own set of unique ID

numbers for that table.

The term relational derives from the fact that such databases use multiple tables to store the

information, with data linked together by relationships between these tables. These tables

contain information about like entities and make storage more efficient by avoiding

duplication. For example, in the car table there is no point in storing both the make and

model of each car. You know that every record having Vectra as the model will also have

Vauxhall as the make. You can store the relationship between model and make in a small

separate table and only record the model in the main table. This is a key concept of database

design and is known as normalisation. Database normalisation saves storage space and

makes the data easier to index and analyse. Querying highly normalised relational databases

can become quite complex since a large number of tables may need to be linked together.

Crucial to the efficient querying of a database is the way in which the tables are indexed . You

can create indexes on tables that enable the computer to sort through and answer a query

quickly. One way in which this can work is for a column containing textual data to generate

an index in which the values are sorted alphabetically. The index stores ranges of values so

that to respond to a query the software only has to search through a small subset of the

whole table to find the required records.



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6.4.3 Object databases

Object-oriented databases have their origins in the realm of object-oriented programming

languages. This subject is notoriously difficult to explain in simple terms and it may be

advisable to seek more detailed resources and devote some time to fully grasp the concepts

of object-orientism.

OODBMS organise information very differently than RDBMS. Rather than spreading the

information about an entity across a range of linked tables, it is stored together in discrete

lumps called objects. Each object is defined within a hierarchy of object classes so that it

inherits properties from a parent class. Additional attributes can be defined within the object

and they are said to exhibit encapsulation because they can self-describe their own particular

set of properties, and therefore the way in which they can be queried. Object-oriented

databases can make it easier to model real-world phenomena in a logical form.

There is a particular problem for the GIS student learning about object databases: the use of

the term object . The situation gets clouded by the fact that geospatial data corresponds to

real-world objects. In GIS the shapes and locations of things are stored as coordinate

geometry. GIS data is often stored in a database, either storing the coordinates as numbers

or using special geometry data types. You will hear GIS practitioners refer to an object

database to describe any database that can store the geometry of topographic objects in its

tables. To a practitioner of pure computer science the distinction between relational and

object databases has nothing to do with geography. To make things worse, there is a hybrid

type of database called object-relational in which advanced data types can be stored in

relational tables that reproduce some of the advantages of the object model. In theory, you

can store object geometry in each of these database types: relational, object andobject-relational.

As you can imagine, it is very important when using this jargon that you know exactly what

you mean by an object! The term spatial database is better for specifying the storage of

geographical features.



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6.4.4 GIS and databases

As previously mentioned, many implementations of GIS use a database rather than simple

files for the data storage. Geographical datasets can be extremely large and so the benefits

of database storage are as applicable to GIS as to any other kind of system. The elements of

data security, the ability to cope with large data volumes and the accessibility to multiple

users are equally important. The explosion in the availability of cheap, high-volume diskspace has fuelled the proliferation of large databases.

Many large organisations maintain vast enterprise-wide information systems that incorporate

different types of geographical objects. For example, a utility company will use GIS to store

information about its pipe networks, the location of its customers and the location of its

maintenance teams. This information will need to be continually updated as it is much easier

to lock the record for a single feature, perform edits and then perform the update if the

features are stored in a database rather than in a file. The database also allows the various

departments to view the information in different ways.

All major GIS software vendors provide tools to enable database storage. Database

management can be a complicated and specialist task, so products are developed to provide

a user interface similar to that of a regular desktop GIS but which also handle the database

administration side. Such products are often referred to as middleware. Middleware is usually

designed to operate across a range of the most popular database products such asMS-Access, SQL Server, Oracle

®, Sybase

®, Ingres

®and IBM

®. More recently, the database

software companies themselves have been producing extensions to the standard

functionality that allow for the storage of complex data types, for example, coordinate

geometry, raster images and terrain models. This is a telling sign of how GIS has become

recognised as a key component of information technology.



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6.4.5 Advanced database technology

Databases are now used in sophisticated ways that go beyond the simple storage of

information and its retrieval by structured queries. Two techniques that are often encountered

in the study of GIS are data warehousing and data mining. Databases are designed as part

of a particular system, only storing the information needed to make that system work.

Organisations end up with many databases serving different functions and often this datacan contain information that has value beyond the purpose of the individual systems for

which they were designed. The centralised gathering together of diverse sets of information

stored within an organisation is known as data warehousing.

Techniques have emerged that automatically scan the information held in a data warehouse

to identify possible relationships between data items. This is known as data mining, which

can reveal phenomena that may otherwise remain undetected. Terms you will hear often

associated with data mining are regression, classification and clustering. Mostly, data mining

concerns a statistical analysis of the contents of the data warehouse to identify

commonalities and patterns. For example, regression refers to the mathematical analysis of

numerical data to identify a formula that best fits the trends in the data. If successful this canenable successful prediction of future results.

Another feature of databases that is very important to their application in GIS is indexing. The

way in which indexes speed up the response to queries has already been described. This

becomes very important when performing geographical search queries as it is possible to

generate spatial indexes that break down the space occupied by features in the table and

sort them into a hierarchy similar to the alphabetical sorting of text values. In response to a

request to find all objects that intersect a polygon, it can be quicker to find a subset near that

polygon first and then analyse each object of this subset more accurately to find those that

actually intersect.

Indexing is important because spatial queries can be very complicated and time consuming.

If you are trying to select all features lying within a county boundary you could be checking

from many thousands of records against the shape with thousands of vertices, a very

convoluted geometric algorithm. Spatial databases that contain features in three dimensions

are starting to be developed – for example, to store a building as a 3-D volumetric object, not

just a planar polygon shape. The generation of spatial indexing for three-dimensional space

presents interesting challenges!

Finally in this section, a mention of another key challenge – the storage of data in the fourth

dimension. GIS databases designed to store information about real-world objects and how

they change over time are called spatio-temporal . To truly reflect real-world changes in data

form, a GIS needs to maintain historical records. The simplest way of achieving this is to

keep copies of the data at intervals to create a series of time slices. More ideal, but harder to

achieve, is to archive each feature every time a change is made to it; this means you can

answer a query for any moment in time.



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6.5 Derived mapping

6.5.1 Generalisation

Map generalisation is the process of reducing the scale and complexity of map detail whilst

maintaining the important elements and characteristics of the location. When creating a mapusing traditional manual techniques, a cartographer aims to achieve a balance between the

amount of real-world information required to make the map useful and avoiding confusion for

the user. This is a time-consuming and expensive process.

GIS has led to the realisation that the efficiency of the cartographer could be increased

through the automation of some of the more time-consuming techniques such as line and

polygon simplification. Current off-the-shelf GIS software packages contain tools that allow

basic generalisation to be performed. An example of polygon generalisation is shown here.

Merging and simplification have been used to produce a cartographic representation of the

original data.

Generalised data Source data

Probably the most famous line generalisation algorithm was developed by Douglas and

Peucker in 1973. The Douglas-Peucker algorithm simply filters the number of vertices along

a digitised line to create a representation suitable for the specified depiction scale.



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Although these algorithms go some way to help in the automated production of smaller-scale

maps, generalisation technologies are very much in their infancy. The challenge of replacing

an experienced cartographer with a computer that can make the same decisions to produce

a map is significant.

The main problem that needs to be addressed in generalisation is how to resolve the conflict

between different map features when they are displayed at smaller scales. As there is notenough space to display all of the information in an uncluttered manner, methods to typify the

data in an intelligent, consistent and coherent way at smaller scales need to be developed. It

is for this reason that the move from map generalisation as a manual art to a computerised

scientific process is a distant dream. For the foreseeable future the process will be a semi-

automated collaboration between cartographer and machine.

6.5.2 Text placement

It is difficult to place text on a map so that it is both legible and clearly associated with the

feature that it is annotating. Text placement refers to the complex challenge of achieving this

in an efficient manner to generate high-quality results. Text can of course be placed bysimple manual methods, although this is a time-consuming and inconsistent process. The

automatic generation and placement of text can result in savings in time and labour together

with a more repeatable result. Although seemingly simple in concept, this automatic process

is remarkably difficult to achieve in practice and is the subject of widespread research

interest.

The manner in which text is placed on a map depends largely upon the cartographic symbols

that are chosen to represent the points, lines and polygons of the source data and how they

relate to each other in a spatial context. One layer’s text or symbols may have a dramatic

impact in determining the placement of another layer’s text. It is therefore necessary to

assemble all the data layers required within the final map, then symbolise their featuresaccording to the map’s extent and scale before text placement takes place.

Buildings and text Roads and text (in red)



The GIS files


• Modern automatic text placement software offers flexible placement options. There are

now choices of font styles, sizes and colours; preferred location of a piece of text;

weightings as to which text is more important and takes preference over other text (for

example, road text might be more important than building text); and minimum and

maximum allowable distances between different labels. A predetermined set of rules can

therefore be created applying to any source data for any location at any given scale. This

results in a map product that is generated more consistently and also more efficiently,thereby greatly reducing the amount of manual effort required in its production. See

below for an example of automatically generated text.

6.5.3 Automated cartography

In Chapter 2 we showed how the appearance of vector data can be readily altered using

symbols and different line styles. In the two previous sections of this chapter we examined

the more advanced concepts of generalisation and text placement. These ideas, and more

besides, are all relevant to the automatic generation of products from source GIS data thatare meaningful and useful to us.

When we talk of automated cartography, what we are trying to achieve is a fundamental GIS

goal of capture once, use many times. In other words, it is inefficient to go through the

process of manually creating an aesthetically pleasing map every time something changes.

It’s far more desirable to automatically represent and display the source data as often as

required and in an infinite variety of ways. Modern GIS software can be used to rapidly and

efficiently generate highly complex maps from basic point, line and polygon features.

Source vector data Derived mapping



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Automated cartography can become a highly sophisticated business. In addition to

overcoming the problems of scale differences and placing text appropriately, we may also

wish to generate different kinds of map for different users. For instance, to create a map

oriented in the direction in which someone is travelling or with colours that are not affected by

one person’s colour blindness.

• Electronic data and electronic displays enable new forms of cartography to bedeveloped. For instance, standard data formats such as Virtual Reality Mark-up

Language (VRML) allow maps to become virtual three-dimensional worlds that you can

explore as if you were flying through the landscape. Furthermore, geographical data is

not necessarily best represented as a map. In many cases we are more interested in

direct information, such as navigation instructions, which might be delivered as text or as

synthesised voice. Where will this lead? Smelly and tasty GIS?

6.5.4 Data from imagery

Imagery – usually from aerial or satellite sensors – is widely used on GIS platforms as abackdrop to vector mapping. An image may contain an abundance of visual information that

is not conveyed by the points, lines and polygons of a vector map. As far as GIS software is

concerned, however, an image is a dumb background. A key research challenge is to derive

vector objects from imagery.

An image is a raster dataset: it is a grid of squares or pixels. Each pixel has a numerical

value that may relate to colour, height or indeed virtually anything measurable.

Numerical values Colour representation



The GIS files


Human interpretation is often used to derive data from imagery. An operator traces lines over

the on-screen image in a technique known as heads-up digitising. This process remains very

labour intensive, however, and significant efforts are being made to find ways to automate it.

Aerial Imagery Digitised Buildings

One approach is to look for abrupt changes or discontinuities in the image that will equate to

a line feature in a map. This can be achieved using an edge detection algorithm that applies

a mathematical function to each pixel and its immediate neighbours in turn. The result is an

image of lines that can simply be converted into vectors. These lines are often very messy,

however, and this method is best used where the discontinuity itself is distinct and separate.

An alternative approach is to use software to look for similar clusters of pixels and thereby

classify the image into distinct areas. Where successful, this will identify real objects such as

buildings, fields and bodies of water within a classified image, which may then be converted

into a vector map. Accurately and appropriately deriving vector data in this way is a complexactivity at the forefront of research. The ability to automatically generate a map from an aerial

photograph or satellite image is a holy grail of GIS because it would help make data far more

inexpensive and up to date.

Imagery Derived Area Map



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The world of the GIS user is also being revolutionised by the Internet. All of the leading GIS

software vendors now have products that adopt the client-server architecture. This means

the software and data resides on a web server and multiple-client applications access the

GIS processing functions across a network. This model is overtaking the use of multiple

desktop software installations, especially in large organisations. It can even work across the

WWW, and there are many sites that do more than just generate location maps. You can

have GIS tools on the web page accessing the full range of GIS functions on the server:selection by attribute, spatial query, thematic mapping, data editing or 3-D visualisation. A

major advantage of this model is that the centralised map data only has to be stored and

maintained in a single location, meaning users are always viewing the most up-to-date

records.

With Internet GIS there is always a trade-off between the sophistication of user tools and

response times. Any system that uses the Internet is constrained by the download speed of

the connection. The smaller the amounts of data being used and the simpler the client user interface, the faster the application. Internet GIS products differ in the way this balance is

approached. Some systems use a very simplistic user front end and display the results of the

server-side process by delivering a simple raster image. This means that the applications

tend to be fast and robust and will work within a standard browser. Other systems require a

client-side plug-in to be downloaded to give richer functionality to the user. Larger amounts of

data can be downloaded from the server to the plug-in, which can make the system work

more slowly; the benefit, however, is a more sophisticated set of user tools and greater

interaction with the data. The choice needs to be made based on the specific requirements of

the application and the expertise of the user community.

6.6.4 Web GIS futures

One of the ultimate goals of the GI industry is to have full interoperability between web-based

geographic datasets enabling information stored at different locations on the web to be

viewed together in single applications (see section 6.3 Standards). With many of the major

current GIS products, not only can you access web-based client server versions of the

software but the standard desktop software can also load files stored centrally on the

Internet. So you could be looking at your locally held map files and then overlay a layer read

from a universal resource location (URL) on the WWW. The full vision of interoperability will

have web-based applications that can read all data files from any location on the Internet,

irrespective of data formats or the software being used.



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As well as reading a data layer from a URL it is also possible to submit specific queries

(requests) and receive back one or more individual elements. This is known as feature

serving. For example, a GIS application could submit a query to a web server requesting

information about a district boundaries layer. The response could be a list of the district

names held in a particular table. If the client then submits a request for a particular district,

the boundary polygon geometry can be returned in response and the individual feature is

served to the client application for display, analysis or download. The OGC is very active in

establishing standards for map and feature serving on the web, see section 6.3 Standards.

The model by which information is exchanged between systems across the Internet, with

small packets of data being returned in response to specific queries, is becoming more

pervasive in all areas of computing. Information providers can establish web services in

which a single data store is created and standard open programming interfaces are

published and made available to system developers.

This means that a system can be developed that can call in exactly the required pieces of

information at exactly the time they are needed, rather than having to maintain many multiple

copies of the same data, which can soon become out of date and degraded. Fuelled by

increased bandwidth, better security and adoption of standards, the Internet has moved from

the periphery to become a fundamental component of real-time IT architectures. The world of

GIS is no exception to this trend and many GI web services are being developed and made

available, replacing the situation of organisations having to obtain large volumes of map data

to manage themselves.



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6.7 Mobile GIS

6.7.1 Positioning

Everyone has experienced the feeling of being lost. Positioning is the process of gaining

information about our location. This can take many different forms and often it is enough toknow which town, street, house number or room. Our location might also be specified in

terms of latitude and longitude, or in metres north and east of a false origin, as in the case of

a national grid.

A Cartesian coordinate system such as in Chapter 2, section 2.4.3 is a grid, with one corner

being the arbitrary ‘false origin’ (0,0) and all positions on the grid measured as distances

north and east of it. British National Grid is one example.

For example, the position of Southampton in British National Grid (BNG) coordinates is

440 000 metres east of the false origin and 100 000 metres north.

Although coordinates are essential, most people are usually more interested in which house

and street to find someone. Coordinates are therefore often linked to a computerised map

showing information that can be interpreted by a user to allow functions such as route

planning and querying the user’s current location.

Many technologies are available for positioning, but perhaps the best known is the US Global

Positioning System (GPS) (See Chapter 2, section 2.3.2). GPS receivers provide location

information as a set of coordinates in latitude and longitude format.



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24 GPS satellites orbit the Earth, providing constant position information to commercially

available receivers on the ground.

The receiver calculates its location from distances measured to satellites orbiting the Earth.

The receiver picks up a digital signal transmitted by the satellites and also measures the time

taken for the signal to arrive. Since the signals travel at the speed of light, the receiver can

calculate how far away the satellite is by calculating distances from at least four satellites and

simultaneously knowing their exact positions in the sky. This gives the position of the receiver

accurate to within about 100 metres anywhere on Earth; however, signals from more

satellites and various techniques can achieve accuracy below one centimetre.

Originally GPS was intended to provide position information for the US military, but today

GPS has a multitude of civilian uses, from surveying to the provision of LBS, which is what

will be discussed in the next section.

6.7.2 Location-based services (LBS)

LBS are a relatively new concept in GIS. Although maps have been around for centuries, a

map that can automatically find your nearest Italian restaurant, plan the most interesting

route there, tell you what is currently on the menu, and allow you to place an order to be

ready on arrival is a major advance. This is the promise of LBS and the reason why it is such

a hot topic today.



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Companies developing such services have had a hard time selling them to customers, and

this may be for many reasons. Two of these may be a lack of understanding of what LBS is

and the cost of the technology involved. The cost is coming down fast, however, and more

and more people are beginning to understand what LBS can do for them. Today LBS

applications are even available on some mobile phones, making them accessible to

everyone.

Future LBS applications might:

Direct you to the nearest service station when your vehicle’s engine management system

detects a problem, warn the station that you are coming so they have the necessary parts

ready for when you arrive, order a taxi for you, and let your colleagues or family know you will

be late.

Replace paper maps with electronic devices that not only allow you to see the area you are

in but pinpoint your position, allow you to select map features and display information about

them, plan routes, guide you with voice commands and recalculate routes automatically if

conditions change.

Alert emergency services the moment a medical problem is detected, and direct them to your

home via the quickest possible route, taking into account traffic conditions and drive

restrictions. It could even alert them to what the problem is so that the appropriate treatment

can be given immediately on arrival. Meanwhile, your calendar appointments could be

automatically rearranged, leaving you worry free during your recovery.

The common theme running through all these suggested applications is the location of the

user. It is this single parameter that defines LBS as a useful technology for the world of

today.

6.7.3 Personal and vehicle navigation

LBS, as discussed in the previous section, rely on information about the environment around

them combined with information about location, to provide users with specific services. One

application of LBS is personal navigation, allowing you to find your way around without

getting lost. Some personal navigation systems are better than others. The most commonly

used navigation devices are a map and a compass; these however, have their limitations.

Maps can be inconvenient when unfolded, only provide as much information as can be

reasonably fitted onto them and cannot be viewed at different scales. The compass only

provides information about direction and then only when the user knows how to use it

correctly.



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Today technology has all but replaced both of these instruments. Mobile devices allow the

user to view maps on screen, and GPS has replaced the compass by providing position,

speed and height data as well as direction information. In-vehicle navigation is one area

these devices have found an immediate niche. Many models on the market now sport in-built

satellite navigation systems, meaning that the car has a built-in GPS system and navigation

screen on the dashboard. Some systems even speak to you in a friendly voice, telling you

when to make a turn, how far you have to go, letting you know that you have taken a wrongturn, and welcoming you to your destination. Only a few years ago such technology was

expensive, not very effective and the stuff of research; today it is commonplace, relatively

cheap and increasingly usable.

In-vehicle satellite navigation systems usually consist of the hardware – comprising GPS,

screen and computer system to drive it – and software – comprising a set of maps and a

computer program usually obtained on CD-ROM. As new roads are built, the CD-ROM can

be updated to the latest version, or, if going abroad, the CD-ROM for the destination country

could be bought. Future systems might be able to use mobile phone technology to download

maps of the area the vehicle is in, in real time from a server, eliminating the need for

CD-ROMs altogether.

6.7.4 LBS for the mass market

As we have seen in the previous sections, LBS have many applications, and their appeal

might lie both in the company environment where such services can be used, for example, to

route delivery vehicles effectively or to help people find their way to their nearest restaurant,

ATM or movie theatre. Once the domain of companies who could pay for it easily, LBS

technology is now accessible to almost everyone, thanks to the mass production of low-cost

mobile devices that almost everyone can afford.

Perhaps LBS will mainly appeal to the mass market because it solves everyday problems.

Since location is such a basic parameter in almost everything we do, a multitude of uses

could be found for LBS; many of these are still waiting to be discovered. There is a huge

potential for creative companies to come up with new and innovative services for everyone to

use their LBS device, be it a Personal Digital Assistant (PDA) or mobile phone. LBS could

save you time and money – even your life.



ArcInfo and ESRI are trademarks of Environmental Systems Research Institute, Inc.

AutoCAD is a registered trademark and DXF a trademark of Autodesk Incorporated. IBM is a

registered trademark of International Business Machines Corporation. Ingres is a registered

trademark of Computer Associates International, Inc. MapInfo is a registered trademark of

MapInfo Corporation. Microsoft is a registered trademark of Microsoft Corporation. OpenGIS

is a registered trademark of Open GIS Consortium, Inc. Oracle is a registered trademark of

Oracle Corporation. Sybase is a registered trademark of Sybase Inc.

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