Some of this material was presented by Bruce Stauffer, Advanced Technology Solutions, Inc., and Todd...

Some of this material was presented by Bruce Stauffer, Advanced Technology Solutions, Inc., and Todd Bacastow, Penn State.

GIS BOOT CAMP

Todd Bacastow


Geography matters!

• ‘Geographic Information’ is information which can be related to specific locations.

• Most human activity depends on geographic information.


Topic 1: What is GIS?

• Dozens of possible definitionsSome emphasise the technology

The Hardware The Software

Others focus on applications Other terms often encountered: LIS,

AM/FM, Geo-information systems, etc. May emphasise different roles for the

system, e.g. spatial decision support system, spatial database system, etc.


One definition of GIS (Dueker and Kjerne,

1989)

• “Geographic Information Systems - A system of hardware, software, data, people, organizations and institutional arrangements for collecting, storing, analysing, and disseminating information about areas of the Earth”


GIS as a tool

• Majority view of GIS

• Focus is on hardware, software and routines

• A technocentric perspective

• The favoured viewpoint of the system vendors


GIS as science

• Emphasis is on data, human uses, contexts

• A more academic perspective

• Geographic information science is the “science behind the systems”

• Includes concepts of spatial reasoning, cognition, human-machine communication, visualisation, data modelling, etc.


GIS is a product of a particular culture

• Most GIS developed in Europe/N. AmericaUSA: Arc/Info, ArcView, Intergraph,

Bentley, Autodesk, MAP, GRASS... Canada: Caris, Spans, GeoVision...France: GeoConcept, Carto 2-D...UK: Smallworld, GIMMS, Laserscan...Netherlands: ILWIS, PC Raster...


GIS is a commercial product

• Developments often driven by commercial considerations, less by scientific ones

• Vendor’s decisions usually based on questions of profitability

• Critical evaluation of proprietary GIS is rare


What GIS is not

• GIS is not simply the technology: it also has a (growing and important) conceptual base

• GIS can not produce good results from bad data or poor conceptual frameworks

• GIS is not simply a program to produce maps

• GIS is not a substitute for thinking! • GIS is not the universal answer to all

problems!


Topic 2: Sources of Spatial Data


Data input - a major bottleneck

• Costs of input often >80% of project costs

• Labor intensive, tedious, error-prone

• Construction of the database may become an end in itself the project may not move on to

analysis of the data collected• Essential to find ways to reduce

costs, maximise accuracy


Sources of digital map data

• National Mapping Organization

• Other government agencies

• Commercial data vendors


Standards

•standards may be set to assure uniformity within a

single data set or across several data sets

ensure the data can be shared across different hardware and software platforms


What if the Data do not exist at all?

• Field data captureMay be done manually (e.g. direct

survey), automatically (e.g. automatic data loggers, etc.) or a combination of the two

• Remote sensingIncludes satellite imagery,

geophysical survey, air photosMay be used as alternative source of

data


Integrating different data sources: issues

• Formatsmany different format standards exist a good GIS can accept and generate

datasets in a wide range of standard formats



• Projections Many ways exist to represent curved surface

of the earth on a flat map Some projections are very common A good GIS can convert data from one

projection to another, or to latitude/longitude Input derived from maps by scanning or

digitizing retains the original map's projection

With data from different sources, a GIS database often contains information in more than one projection, and must use conversion routines if data are to be integrated or compared



• Scaledata may be input at a variety of

scalesscale is an important indicator of

accuracymaps of the same area at different

scales will often show the same features

variation in scales can be a major problem in integrating data



• Resampling rastersRaster data from different sources

may use different pixel sizes, orientations, positions, projections

Resampling is the process of interpolating information from one set of pixels to another

Resampling to larger pixels is comparatively safe, resampling to smaller pixels is very dangerous


Topic 3: Representing Spatial Entities


Representing Spatial Entities

• The object-focused approachBased on recognition of discrete

objects or entitiesMay be layer-based or object-

orientedUsually represented by Vector GIS


Two ways of representing space in a GIS

• The Tesseral (field-oriented) approachTypically seen in Raster GISAlso in some other models


Vector data models

• Based on the recognition of discrete objects or entities

• The location/boundaries of these objects defined with respect to some coordinate system

• Emphasis is on boundaries, space within and between boundaries implied

• Objects are usually defined in terms of points, lines and areas

• Complex graphic objects are seen as amalgamations of simpler ones

• Typical Vector GIS include ARC/INFO, MapInfo Intergraph MGE


The vector data model

• Sequences of points can be used to define lines

• Lines themselves can be aggregated to represent Networks Boundaries of polygons and regionsTopographic features (contours,

breaks of slope, etc.).


Topology

• An essential element of vector GIS

• A distinct branch of mathematics

• Defines spatial relationships between objectsAdjacency, connectivity,

containment, etc.

• Essential for most vector GIS operations


Advantages and disadvantages of the vector

approach• Lower data volumes

• More adaptable to variations in scale/resolution of phenomena

• Tends to be more suited to social and economic applications

• Disadvantages: Less adaptable to uncertainty, fuzziness Often no “lowest common denominator” of

aerial unit .


Objects versus layers

• Major point of discussion in GIS since mid-1980s

• Alternative strategies for vector representation of geographic space a “stacked” sequence of layers a collection of discrete objects

• Difference in how contents of the database represents the real world

• Echoes wider developments in Computer Science


The Object view

• More closely mirrors natural ways of seeing the world

• Objects usually used in speaking, writing, thinking about the world

• Objects are fundamental to our understanding of geography

• Object-oriented approaches may offer data storage and processing advantages


What are these objects?

• Graphics objects can be points, lines, areas

• Geographic objects can be roads, houses, hills, etc.

• A space can be occupied by many, or no, objects A river is an object (has an identity, name,

coordinates, properties, etc.) A line is an object (also has an identity,

name, coordinates, properties, etc.)


Applications of object view:

• Utilities and facilities managementConcept of empty space littered with objects

fits many needs of managing infrastructureTwo or more objects may occupy same

horizontal position, separated verticallySmaller objects may be part of larger ones

(e.g. pipes as part of networks) and vice versa

Idea of a variable measured everywhere on Earth has little relevance


The Layer view

• Locations specified by a system of coordinates

• Geography of real world conceptualised as a series of variables (soils, land use, elevation, etc.)

• Each layer in the database represents a particular variable


The Layer view

• Layer view often more compatible with theories of atmospheric, ocean processes

• Object view is less compatible with concept of continuous change

• Good for resource management applications

• Much data for environmental modelling derived from remote sensing Implies a layer view


Disadvantages

• The layer approach usually requires many different files to represent each layer

• Some files contain the actual data

• Some contain registration information

• Some contain topological information to construct complex geometries from more primitive ones


Applications of layer view

• Resource managementgeographic variation can be

described by relatively small amount of variables

conceptualisation reasonably constant between scales

movement of individuals can lead to difficulties of representation and tracking across layers


Tesseral approaches to GIS


Tesseral geometries

• From the Greek, tetara or Latin tessella = a tile

• Tessallations are “sets of connected discrete two-dimensional units” thus mosaics or tilings of space

• May be regular or irregular

• Focus is on space occupancy

• Emphasis is on areas, boundaries are implied


Types of tessallation

• Regular tessalationsRasters

• Irregular tessalationsQuadtreesVoronoi TessalationsTriangulated Irregular Networks

(TINs)


The raster

• “Raster Data are spatial data expressed as a matrix of cells or pixels, with spatial positioning implicit in the ordering of the pixels” (AGI 1994)

• Raster data structure widely used in GIS e.g. IDRISI, GRASS, Arc/Info’s GRID module


Why use rasters?

• Raster data from other disciplines

• Ideal for representing continuous variations in space

• Common way of structuring digital elevation data

• Assumes no prior knowledge of the phenomenon

• Uniform, regular sampling of reality


Why use rasters?

• Often used as common data exchange format

• Raster algorithms often simpler and faster

• Easy to program, less need for special hardware

• Raster systems tend to be cheaper than vector


Issues and trade-offs

• May give very large data files typical raster databases may contain >

100 layers each layer typically contains hundreds or

thousands of cells

• Many options exist for storing raster data

some are more economical than others in terms of storage space

some more efficient in terms of access and processing speed


Issues and trade-offs

• Maximum resolution determined by the size of grid

• Less easy to connect tabular (attribute) data to spatial objects

• Raster data lack topology

• Regular geometry of raster cells may not accurately reflect the variations of reality


Variable-resolution tessalations

• Triangulated Irregular Networks (TINs)Alternative to regular raster for terrain

modellingDeveloped in 1970s Can build surfaces from irregular arrays of

point elevation dataMany commercial GIS now offer TIN

capabilities.


Topic 4: Coordinates, Datums, and Projections


Spherical CoordinatesSpherical “grid” is called a graticuleLatitude references north and southLongitude references east/westLine of constant latitude is a parallelLine of constant longitude is a meridianMeridians converge at the poles

Latitude range: 0 to 90 degrees north and southLongitude range: 0 to 180 degrees east and west

0º LatitudeP

rim

e M

erid

ian

0º

Lo

ng

itu

de

Equator

90º N Latitude

90º S Latitude

SouthernHemisphere

NorthernHemisphere

EasternHemisphere

WesternHemisphere

90º W Longitude

0º L

on

git

ud

e

1

80º

Lo

ng

itu

de

90º E Longitude


Spherical CoordinatesA spherical coordinate measure is expressed in degrees (º), minutes (‘) and seconds (“)

1º = 60’ = 3,600” ; 1’ = 60”

Expressed as:ddd mm ss N/S, ddd mm sss E/W

Note the convention is to express latitude (y) before longitude (x), but computer environments use x,y

• In most digital environments, degrees, minutes and seconds are converted to decimal degrees: degrees + (min/60) + (sec/3600)• Harrisburg International Airport is: 40º12’N, 76 º45’W, or40.20N, 76.75W


Spherical Coordinates

Eastern and Northern Hemisphere: +x, +y

Eastern and SouthernHemisphere:+x, -y

Western and Northern Hemisphere: -x, +y

Western and SouthernHemisphere:-x, -y


Cartesian Coordinates

X axis

Y a

xis

0,0 1 2 3 4 5 6 7 8 9

1

2

3

4

5

6

(2.0,3.0)

(4.5, 4.5)

(7.0,2.0)


Horizontal Datum

North American Datum of 1983• an earth centered datum where the center of

the spheroid is the center of the earth • based on the Geodetic Reference System of

1980 (GRS80): a better approximation of earth’s true size and shape.

• twice as accurate as the NAD27: resulted in controls shifted up to 100 meters

North American Datum of 1927• A local datum centered on the Meades Ranch

in Kansas. Surface of ellipsoid was tangent to the Meades Ranch

• 300,000 permanent control network

• Clarke 1866 spheroid used to define the shape and size of the earth

Meades RanchKansas

EarthCenter

Clarke 1866Center

Clarke 1866 Spheroid

GRS80 Spheroid

Meades RanchKansas

EarthCenter

Clarke 1866Center

Clarke 1866 Spheroid

GRS80 Spheroid

NAD 1927 DATUM

NAD 1983 DATUM


Vertical Datum

North American Vertical Datum of 1988

• 1929 datum adjusted based on more precise measurements of geoid shape and mean sea levels.

• some bench mark heights changed up to 2 meters, but heights between adjacent benchmarks changed < a few millimeters

• provides better geoid height definitions in order to convert earth centered GPS derived heights

National Geodetic Vertical Datum of 1929

• vertical datum based mean sea level as determined by years of observations at tidal gauging stations

• 585,000 permanently monumented vertical benchmarks interconnected by leveling

Vertical Datum(mean sea level)

Land Mass

Sea Floor

Sea Level


Projections

To represent a spherical model of the earth on a flat plane requires a map projection!

Projection


Map ProjectionsZ = rotational axis

Y

X

o a

b

a

Spheroid: a three-dimensional geometric surface generated by rotating an ellipse about one of its axes.

It provides an approximate model of the earth’s shape, the first step in constructing a projection


Map Scale• Options to deal with minimum mapping unit

size at desired design scaleAdopt a larger map scale for the source

Increased cost for acquisition Increased storage for larger data volume

Convert area features to points or lines Evidence of feature is retained Inconsistency in feature representation May give up desired metrics (area, perimeter) May give up overlay analysis options

Eliminate small areas Consistency in feature representation No evidence of omitted features


PennsylvaniaStatewide Projection

• Projection: Lambert Conformal Conic• Spheroid: GRS80• Central Meridian: 77º 45’ 00.0” W (-77.75)• Standard parallels: 40º 36’ 10.8” N (40.603) 41º 16’ 33.6” N (41.276)• Reference latitude: 39º 19’ 59.9’ N (39.333)

Considerations for selecting a statewide projection for Pennsylvania:• Pennsylvania’s east/west extent is best suited for a conic projection • If you need to preserve area, use Alber’s Equal Area Conic • If you need to shape and angle, use Lambert Conformal Conic • Select two standard parallels that divide the state into approximately even

thirds north to south • Select a central meridian that divides the state approximately into equal halves


Map Projections

• Transform spherical geographic space to a 2-D planar surface.If it is a map, it has been projected!Eliminates need to carry a globe around in

the pocket!2-D Cartesian coordinate space is better suited

than spherical coordinates when conducting traditional surveys, mapping, and ground measurements.

• Ensures a known relationship between map location and earth location


Map Projections

CYLINDRICAL PLANARCONIC


Map Projection Distortion• Conformal projections

Preserve relative angle and shape for small areas, but area is very distorted

For any given point, local scale is constant in all directions

Used for navigation, meteorological charts Examples: Mercator and Lambert Conformal Conic

• Equivalent projections Preserve area but shape and angles are very distorted. A coin placed at any location on the map covers the

same amount of area Use when area conveys meaning (thematic maps

showing density) Examples: Albers Conic Equal Area and Peters

Projection


Map Projection Distortion

MERCATOR (Conformal)

ROBINSON

PETERS (Equivalent)


Map Scale

• Map scale: the relationship between map distance (or display distance) and actual ground distance

• Scale Calculations:Scale = map distance / (ground distance x conversion

factor)To determine map scale when map and ground

distances are known: 2.5” on map = 500 feet on ground 2.5/500*12 = 2.5/6,000 = 1:2,400

To determine ground distance when map scale is known:

1:4,800 is same as 1” = 4,800” 1.82” on map: 1 * 1.82 = 4,800*1.82 1.82” = 8,7376” = 728’


Map Scale• Small Scale Maps

Large denominator in RF (1:14,000,000)Maps of continents and world maps

• Medium Scale MapsMedium denominator in RF (1:24,000)USGS Topographic Quadrangles

• Large Scale MapsSmall denominator in RF (1:2,400)Tax maps, utility maps

• The smaller the number in the denominator, the larger the map scale½ is “larger” than ¼ and ¼ is “smaller” than ½

Some of this material was presented by Bruce Stauffer, Advanced Technology Solutions, Inc., and Todd...

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