Geographic Information SystemsCIVE 1188

Hanadi Rifai, PhD, PE

Civil and Environmental Engineering

University of Houston

Where did GIS come from?

• GIS is built upon knowledge from geography, cartography, computer science and mathematics

• Geographic Information Science is a new interdisciplinary field built out of the use and theory of GIS

Defining GIS

• Different definitions of a GIS have evolved in different areas and disciplines

• All GIS definitions recognize that spatial data are unique because they are linked to maps (Space matters!)

• A GIS at least consists of a database, map information, and a computer-based link between them

Spatial and non-spatial data

Crimes during 1995Date Location Type1/22 123 James St. Robbery1/24 22 Smith St. Burglary2/10 9 Elm St. #4A Assault2/13 12 Fifth Avenue Breaking

& Entering

Part No. Quant. Desc.1034161 5 Wheel spoke1051671 1 Ball bearing1047623 6 Wheel rim1021413 2 Tire1011210 3 Handlebars

Figure 1.1 Two databases. A database contains columns (att ributes) and rows (records). The bicycleparts list on the left is not spatial. The parts could be located anywhere. The list of crimes on the rightis spatial because one of the at tributes, the street address, locates the crimes on a map. This list couldbe used in a GIS.

Map Overlay

Figure 1.3 Map overlay as presented in Design with Nature by Ian McHarg. Each transparent layer map“blacked out” areas excluded as unsuitable locations.

SOILS

PARKS

URBAN

SOLUTION MASK

FOREST

The Feature Model

BM 123

r

L a k e

Figure 1.2 The Feature Model: Examples of a point feature (elevation bench mark), a line feature (riv-er) and an area feature (lake).

POINT LINE AREA

FEATURES

A Brief History of GIS• GIS’s origins lie in thematic cartography (manual map overlay) • Computer cartography advances in 1950s and 1960s• Early influential data sets were the World Data Bank and the

GBF/DIME files. Early systems were CGIS, MLMIS, GRID and LUNR

• The Harvard University ODYSSEY system was influential due to its topological arc-node (vector) data structure

• GIS was significantly altered by (1) the PC and (2) the workstation • During the 1980s, new GIS software could better exploit more

advanced hardware • User Interface developments led to GIS's vastly improved ease of use

during the 1990s• During the 1980s, new GIS software could better exploit more

advanced hardware

Sources of Information on GIS

• The amount of information available about GIS can be overwhelming

• Sources of GIS information include journals and magazines, books, professional societies, the World Wide Web, and conferences

• GIS has Web Home pages, network conference groups, professional organizations, and user groups

• Most colleges and universities now offer GIS classes in geography departments

Organizing Data and Information

• Information can be organized as lists, numbers, tables, text, pictures, maps, or indexes.

• Clusters of information called data can be stored together as a database.

• A database is stored in a computer as files.

Flat File Database

Record Value Value Value

Attribute Attribute Attribute

Record Value Value Value

Record Value Value Value

Geographic Coordinates as Data

Building complex features• Simple geographic features can be used to

build more complex ones. • Areas are made up of lines which are made up

of points represented by their coordinates.• Areas = {Lines} = {Points}

Areas are lines are points are coordinates

Basic properties of geographic features

GIS Analysis

• Much of GIS analysis and description consists of investigating the properties of geographic features and determining the relationships between them.

• Two types of systems:– Raster or pixel based– Vector or arc based

RASTER

• A grid or raster maps directly onto a programming computer memory structure called an array.

• Grids are poor at representing points, lines and areas, but good at surfaces.

• Grids are good only at very localized topology, and weak otherwise.

• Grids are a natural for scanned or remotely sensed data.• Grids suffer from the mixed pixel problem.• Grids must often include redundant or missing data.• Grid compression techniques used in GIS are run-length encoding

and quad trees.

Raster Data Formats

• Most raster formats are digital image formats.

• Most GISs accept TIF, GIF, JPEG or encapsulated PostScript, which are not georeferenced.

• DEMs are true raster data formats.

The Vector Model

• A vector data model uses points stored by their real (earth) coordinates.

• Lines and areas are built from sequences of points in order.

• Lines have a direction to the ordering of the points.

• Polygons can be built from points or lines.

• Vectors can store information about topology.

VECTOR• At first, GISs used vector data and cartographic spaghetti

structures.• Vector data evolved the arc/node model in the 1960s.• In the arc/node model, an area consist of lines and a line

consists of points.• Points, lines, and areas can each be stored in their own files,

with links between them.• The topological vector model uses the line (arc) as a basic

unit. Areas (polygons) are built up from arcs.• The endpoint of a line (arc) is called a node. Arc junctions are

only at nodes.• Stored with the arc is the topology (i.e. the connecting arcs

and left and right polygons).

TOPOLOGY

• Topological data structures dominate GIS software.

• Topology allows automated error detection and elimination.

• Rarely are maps topologically clean when digitized or imported.

• A GIS has to be able to build topology from unconnected arcs.

• Nodes that are close together are snapped.

• Slivers due to double digitizing and overlay are eliminated.

Basic arc topology

n1

n2

123 A

B

Arc From To PL PR n1x n1y n2x n2y1 n1 n2 A B x y x y

Topological Arcs File

Figure 3.5 A topological structure for the arcs.

Finding Existing Map Data

• Map libraries

• Reference books

• State and local agencies

• Federal agencies

• Commercial data suppliers e.g. GDT, Thompson, ETAK

• World Wide Web

• GIS vendors package data with products.

USGS: National Mapping

Terrain dataDEMDLG

ContoursDCW

Contours

U.S. Bureau of the Census

NOAA Weather and other data

• Distributed active archive center

• Sioux Falls, SD

• Operated by USGS

Eros Data Center

Digitizing• Captures map data

by tracing lines from a map by hand

• Uses a cursor and an electronically-sensitive tablet

• Result is a string of points with(x, y) values

Scanning

• Places a map on a glass plate, and passes a light beam over it

• Measures the reflected light intensity

• Result is a grid of pixels

• Image size and resolution are important

• Features can “drop out”

Field data collection

Attribute data

• Logically can be thought of as in a flat file

• Table with rows and columns• Attributes by records• Entries called values

Database Management Systems• Data definition module sets constraints on the

attribute values• Data entry module to enter and correct values• Data management system for storage and retrieval• Legal data definitions can be listed as a data

dictionary• Database manager checks values with this

dictionary, enforcing data validation.

Retrieval Operations

Searches by attribute: find and browse.

Data reorganization: select, renumber, and sort.

Compute allows the creation of new attributes based on calculated values.

The Big Eight

• Form the bulk of operational GIS in professional and educational environments

• There are some significant differences between these “big eight” systems.

Arc/Info

ESRIRedlands, CAArc/InfoMarket leaderworkstation (mostly)remarkable functionalitymany formats supportedArcEditArcGRIDArcPlotINFO

ArcView

Versions 1-3, 3.1PC WindowsAvenueWeb linksMap ObjectsExtensions

AutoCAD MAP

Windows all versionsSQL DBF AccessExtension to AutoCADMenu-basedMassive installed baseAdded grid, projection & topology supportDB links good.3D links good

GRASS

First UNIX GISDeveloped by ArmyCorps of EngineersUNIX functionalityMany unique functionsFree until recentlyMany data setsBaylor University now supports

IDRISI

Developed at ClarkUniversity, Worcester MAOriginal in PASCAL, withopen codeDevelopment uses a specialtyWindows/DOSSpatial analysis/stats extensions

Maptitude

Caliper CorporationConsultancyTRANSCAD and GIS+Many network solutionsWindowsImport/ExportAddress matching

Microstation MGE

CAD software with GISextensionsIntergraph Corp, Huntsville ALUses Windows NTMany parcel applicationsWeb extensions, server tools etc.

MapInfo

Based in Troy, NYMapping functionsLimited GIS functionalityUses Visual BasicMany applicationsFavored for 911, field