Geographic information system

A geographic information system (GIS) isspecialized forman information system. Instrictest sense,iscomputer system capableassembling, storing, manipulating,displaying geographically-referenced information, i.e. data identified accordingtheir locations. Practitioners also regardtotal GIS as including operating personnel anddata that go intosystem.

Geographic information systems technology can be usedscientific investigations, resource managementdevelopment planning. For example,GIS might allow emergency plannerseasily calculate emergency response times inevent ofnatural disaster, orGIS might be usedfind wetlands that need protection from pollution.

Tablecontents

1 Relating information from different sources
2 Data Capture
3 Data integration
4 Projectionregistration
5 Data structures
6 Data modeling
7 What's special aboutGIS?
8 Information retrieval
9 Topological modeling
10 Networks
11 Overlay
12 Data output
13 ApplicationsGIS
14 GIS through history
15 Graphic display techniques
16 The futureGIS

16.1 Global ChangeClimate History Program
16.2 Addingelementtime

17 External links

Relating information from different sources

If you could relate information aboutrainfallyour stateaerial photographsyour county, you might be abletell which wetlands dry up at certain times ofyear. A GIS, which can use information from many different sourcesmany different forms, can helpsuch analyses. The primary requirement forsource data consistsknowinglocations forvariables. Location may be annotated by x,y,z coordinateslongitude, latitude,elevation, or by such systems as ZIP codes or highway mile markers. Any variable that can be located spatially can be fed intoGIS. Several computer databases that can be directly entered intoGISbeing produced by government agenciesnon-government organizations. Different kindsdatamap form can be entered intoGIS.

A GIS can also convert existing digital information, which may not yet bemap form, into formscan recognizeuse. For example, digital satellite images generated through remote sensing can be analyzedproducemap-like layerdigital information about vegetative covers.

Likewise, census or hydrologic tabular data can be convertedmap-like form, serving as layersthematic information inGIS.

Data Capture

How canGIS useinformation inmap? Ifdatabe usednot alreadydigital form, that is, informcomputer can recognize, various techniques can captureinformation. Maps can be digitized, or hand-traced withdigitizer,collectcoordinatesfeatures.

Electronic scanning devices will also convert map linespointsdigits.

A GIS can be usedemphasizespatial relationships amongobjects being mapped. Whilecomputer-aided mapping system may representroad simply asline,GIS may also recognize suchroad asborder between wetlandurban development, or aslink between Main StreetBlueberry Lane.

Data capture - puttinginformation intosystem - consumes much oftimeGIS practitioners. Identities ofobjects onmap must be specified, as well as their spatial relationships. Editingautomatically captured information can also prove difficult. Electronic scanners record blemishes onmap just as faithfully asrecordmap features. For example,fleckdirt might connect two lines that should not be connected. Extraneous data must be edited, or removed fromdigital data file.

Data integration

A GIS makespossiblelink, or integrate, information thatdifficultassociate through any other means. Thus,GIS can use combinationsmapped variablesbuildanalyze new variables.

Using GIS technologywater-supplier billing information, itpossiblesimulatedischargematerials inseptic systems inneighborhood upstream fromwetland. The bills show how much waterused at each address. The amountwatercustomer uses will roughly predictamountmaterial that will be discharged intoseptic systems, so that areasheavy septic discharge can be located usingGIS.

Projectionregistration

A property ownership map andsoils map might show data at different scales. Map information inGIS must be manipulated so thatregisters, or fits,information gathered from other maps. Beforedigital data can be analyzed,may haveundergo other manipulations - projection conversions,example - that integrate them intoGIS.

Projection isfundamental componentmap making. A projection ismathematical meanstransferring information fromEarth's three-dimensional curved surface totwo-dimensional medium - paper orcomputer screen. Different projectionsuseddifferent typesmaps because each projection particularly suits certain uses. For example,projection that accurately representsshapes ofcontinents will distort their relative sizes.

Since much ofinformation inGIS comes from existing maps,GIS usesprocessing power ofcomputertransform digital information, gathered from sourcesdifferent projections, tocommon projection.

Data structures

Canproperty ownership map be related tosatellite image,timely indicatorland uses? Yes, but since digital datacollectedstoredvarious ways,two data sources may not be entirely compatible. SoGIS must be ableconvert data from one structureanother.

Image data fromsatellite that has been interpreted bycomputerproduceland-use map can be "read into"GISraster format. Raster data files consistcolumnsrowsuniform cells coded accordingdata values.

Raster data files can be manipulated quickly bycomputer, but theyoften less detailedmay appear less visually appealing than vector data files, which can approximateappearancemore traditional hand-drafted maps. Vector digital data have been captured as points, lines (a seriespoint coordinates), or areas (shapes bounded by lines). In addition, raster data files require some meanstying them toEarth's surface; these vary from vendorvendor, butwidely-used waydoing thisthroughWorld file which representslocation, scalerotation ofmap image.

An exampledata typically held invector file would beproperty boundaries forhousing subdivision.

Data restructuring can be performed byGISconvert data into different formats. For example,GIS may be usedconvertsatellite image map tovector structure by generating lines around all cells withsame classification, while determiningcell spatial relationships, such as adjacency or inclusion.

ThusGIS can be usedanalyze land use informationconjunctionproperty ownership information.

Data modeling

Itdifficultrelate wetlands mapsrainfall amounts recorded at different points such as airports, television stations,high schools. A GIS, however, can be useddepict two-three-dimensional characteristics ofEarth's surface, subsurface,atmosphere from information points.

For example,GIS can quickly generatemaplines that indicate rainfall amounts.

Suchmap can be thoughtasrainfall contour map. Many sophisticated methods can estimatecharacteristicssurfaces fromlimited numberpoint measurements. A two-dimensional contour map created fromsurface modelingrainfall point measurements may be overlaidanalyzedany other map inGIS coveringsame area.

What's special aboutGIS?

The way mapsother data have been stored or filed as layersinformation inGIS makespossibleperform complex analyses.

Information retrieval

What do you know aboutswampy area atendyour street? WithGIS you can "point" atlocation, object, or area onscreenretrieve recorded information aboutfrom off-screen files.

Using scanned aerial photographs asvisual guide, you can askGIS aboutgeology or hydrology ofarea, or even about how closeswamp isend ofstreet. This kindanalytic function allows youdraw conclusions aboutswamp's environmental sensitivity.

Topological modeling

Inpast 35 years, were there any gas stations or factories operating next toswamp? any within two milesuphill fromswamp? A GIS can recognizeanalyzespatial relationships among mapped phenomena. A GIS can determine conditionsadjacency (what adjoins what), containment (what encloses what),proximity (how close something issomething else).

Networks

If allfactories nearwetland were accidentallyrelease chemicals intoriver atsame time, howwouldtake fordamaging amountpollutantenterwetland reserve? A GIS can simulateroutematerials alonglinear network. Itpossibleassign values such as directionspeed todigital stream"move"contaminants throughstream system.

Overlay

Using mapswetlands, slopes, streams, land use,soils,GIS might producenew map layer (or overlay) that rankswetlands accordingtheir relative sensitivitydamage from nearby factories or homes.

Data output

A critical component ofGISits abilityproduce graphics onscreen or on paper that conveyresultsanalysis topeople who make decisions about resources. Wall mapsother graphics can be generated, allowingviewervisualizethereby understandresultsanalyses or simulationspotential events. Internet Map Servers facilitate distributiongenerated maps viaweb technology.

ApplicationsGIS

GIS through history

35,000 years ago, onwallscaves near Lascaux, France, Cro-Magnon hunters drew pictures ofanimalshunted. Associated withanimal drawingstrack linestallies thoughtdepict migration routes. These early records followedtwo-element structuremodern geographic information systems:graphic file linkedan attribute database.

Today, biologists use collar transmitterssatellite receiverstrackmigration routescariboupolar bearshelp design programsprotectanimals. One GIS displayedmigration routes by different colorseach month21 months. Researchers then usedGISsuperimposemigration routes on mapsoil-development plansdeterminepotentialinterference withanimals.

Graphic display techniques

Traditional mapsabstractions ofreal world,samplingimportant elements portrayed onsheetpapersymbolsrepresent physical objects. People who use maps must interpret these symbols. Topographic maps showshapeland surfacecontour lines. The actual shape ofland can be seen only inmind's eye. Graphic display techniques inGIS can make relationships among map elements visible, heightening one's abilityextractanalyze information.

Two typesdata were combined inGISproduceperspective view orportionSan Mateo County, California. The digital elevation model, consistingsurface elevations recorded on30-meter horizontal grid, shows high elevations as whitelow elevation as black.

The accompanying Landsat Thematic Mapper image showsfalse-color infrared image ofsame area30-meter pixels, or picture elements.

A GIS was usedregistercombinetwo imagesproducethree-dimensional perspective view looking downSan Andreas Fault.

The futureGIS

Many disciplines can benefit from GIS techniques. An active GIS market has resultedlower costscontinual improvements inhardwaresoftware componentsGIS. These developments will,turn, result inmuch wider application oftechnology throughout government, business,industry.

Global ChangeClimate History Program

Maps have traditionally been usedexploreEarth andexploit its resources. GIS technology, as an expansioncartographic science, has enhancedefficiencyanalytic powertraditional mapping. Now, asscientific community recognizesenvironmental consequenceshuman activity, GIS technologybecoming an essential tool ineffortunderstandprocessglobal change. Various mapsatellite information sources can combinemodes that simulateinteractionscomplex natural systems.

Throughfunction known as visualization,GIS can be usedproduce images - not just maps, but drawings, animations,other cartographic products. These images allow researchersview their subjectsways that literally never have been seen before. The images oftenequally helpfulconveyingtechnical conceptsGIS study-subjectsnon-scientists.

Addingelementtime

The condition ofEarth's surface, atmosphere,subsurface can be examined by feeding satellite data intoGIS. GIS technology gives researchersabilityexaminevariationsEarth processes over days, months,years. As an example,changesvegetation vigor throughgrowing season can be animateddetermine when drought was most extensive inparticular region. The resulting graphic, known asnormalized vegetation index, representsrough measureplant health.

Workingtwo variables over time will allow researchersdetect regional differences inlag betweendeclinerainfallits effect on vegetation.

GIS technology andavailabilitydigital data on regionalglobal scales enable such analyses. The satellite sensor output usedgeneratevegetation graphicproduced byAdvanced Very High Resolution Radiometer or AVHRR. This sensor system detectsamountsenergy reflected fromEarth's surface across various bands ofspectrumsurface areasabout 1 square kilometer. The satellite sensor produces images ofparticular location onEarth twiceday. AVHRRonly onemany sensor systems usedEarth surface analysis. More sensors will follow, generating ever greater amountsdata.

GISrelated technology will help greatly inmanagementanalysisthese large volumesdata, allowingbetter understandingterrestrial processesbetter managementhuman activitiesmaintain world economic vitalityenvironmental quality.