Geographic Information System.docx
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Geographic Information System.docx
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GeographicInformationSystem
GeographicInformationSystem
1.WhatisaGIS?
Ageographicinformationsystem(GIS)isacomputer-basedtoolformappingandanalyzingphenomenathatexistandhappenonthesurfaceoftheEarth. itiscapableofintegrating,storing,editing,analyzing,anddisplayinggeographically-referencedinformation(spatialdataandassociatedattributes).
GIStechnologyintegratescommondatabaseoperationssuchasqueryandstatisticalanalysiswithuniquevisualizationofferedbymaps. TheseabilitiesdistinguishGISfromotherinformationsystemsandmakeitvaluabletoawiderangeofpublicorganizationsforexplainingevents,predictingoutcomes,andplanningstrategies.
Mapmakingandgeographicanalysisarenotnew,butaGISperformsthesetasksbetterandfasterthanconventionalmanualmethods. BeforeGIStechnology,onlyafewpeoplehadtheskillsnecessarytousegeographicinformationtohelpwithdecisionmakingandproblemsolving.
2.Historyofdevelopment
35,000yearsago,onthewallsofcavesnearLascaux,France,Cro-Magnonhuntersdrewpicturesoftheanimalstheyhunted.Associatedwiththeanimaldrawingsaretracklinesandtalliesthoughttodepictmigrationroutes.Theseearlyrecordsfollowedthetwo-elementstructureofmoderngeographicinformationsystems:
agraphicfilelinkedtoanattributedatabase.
Inthe18thcentury,modernsurveyingtechniquesfortopographicmappingwereimplemented,alongwithearlyversionsofthematicmapping,e.g.,forscientificorcensusdata.
AnotableexampleofthisisJohnSnow’s1854mapdepictingacholeraoutbreakinLondon,whichprovidedanalysistonarrowthesourceofthecholeratoacontaminatedpump,stemmingtheoutbreak.ImagesofJohnSnow’smaps(Fig.1).
Fig.1JohnSnow’scholera-map
From
Theearly20thcenturysawthedevelopmentofphotolithographywheremapswereseparatedintolayers.Computerhardwaredevelopmentspurredbynuclearweaponresearchwouldleadtogeneralpurposecomputermappingapplicationsbytheearly1960s.
Theyear1967sawthedevelopmentoftheworld’sfirsttrueoperationalGISinOttawa,OntariobythefederalDepartmentofEnergy,MinesandResources.DevelopedbyRogerTomlinson,itwascalledCanadianGIS(CGIS)andwasusedtostore,analyseandmanipulatedatacollectedfortheCanadaLandInventory(CLI),aninitiativetodeterminethelandcapabilityforruralCanadabymappinginformationaboutsoils,agriculture,recreation,wildlife,waterfowl,forestry,andlanduseatascaleof1:
250,000.Aratingclassificationfactorwasalsoaddedtopermitanalysis.
CGISwastheworld’sfirstsystemandwasanimprovementovermappingapplicationsasitprovidedcapabilitiesforoverlay,measurement,digitizing/scanning,supportedanationalcoordinatesystemthatspannedthecontinent,codedlinesasarcshavingatrueembeddedtopology,anditstoredtheattributeandlocationalinformationinseparatefiles.Itsdeveloper,geographerRogerTomlinson,hasbecomeknownasthefatherofGIS.
CGISlastedintothe1990sandbuiltthelargestdigitallandresourcedatabaseinCanada.Itwasdevelopedasamainframebasedsysteminsupportoffederalandprovincialresourceplanningandmanagement.Itsstrengthwascontinent-wideanalysisofcomplexdatasets.TheCGISwasneveravailableinacommercialform.ItsinitialdevelopmentandsuccessstimulatedvariouscommercialmappingapplicationsbeingsoldbyvendorssuchasIntergraph.
Thedevelopmentofmicro-computerhardwarespurredvendorssuchasESRI,MapInfoandCARIStosuccessfullyincorporatemanyoftheCGISfeatures,combiningthefirstgenerationapproachtoseparationofspatialandattributeinformationwithasecondgenerationapproachtoorganizingattributedataintodatabasestructures.
The1980sand1990sindustrygrowthwerespurredonbythegrowinguseofGISonUnixworkstationsandthepersonalcomputer.Bytheendofthe20thcentury,therapidgrowthinvarioussystemshadbeenconsolidatedandstandardizedonrelativelyfewplatformsanduserswerebeginningtoexporttheconceptofviewingGISdataovertheInternet,requiringdataformatandtransferstandards.
3.WhatmakesupaGIS?
AworkingGISintegratesfivekeycomponents:
hardware,software,data,people,andmethods(Fig.2).
HardwareisthecomputeronwhichGISsoftwareprovidesthefunctionsandtoolsneededtostore,analyzeanddisplaygeographicinformation.
Geographicdata(layers)andrelatedtabulardataarenecessaryinorderto
performworkinaGISsoftware.
GIStechnologyisforlimitedvaluewithoutthepeoplewhomanagethesystemanddevelopplansforapplyingittoreal-worldproblems.
Fig.2GISkeycomponents
Lastly,asuccessfulGISoperatesaccordingtowell-designedplansandbusinessmethods,whicharethemodelsandoperatingpracticesuniquetoeachorganization.
GISLayer:
AGISstoresinformationabouttheworldasacollectionoflayersthatcanbelinkedtogetherbygeography(Fig.3). Thissimplebutextremelypowerfulandversatileconcepthasproven
invaluableforsolvingmanyreal-worldproblemssuchasthetrackingofdeliveryvehicles,recordingdetailsofplanningapplications,aswellasmodelingglobalatmosphericcirculation.
Fig.3GISLayer
4.TechniquesusedinGIS
4.1Relatinginformationfromdifferentsources
Ifyoucouldrelateinformationabouttherainfallofyourstatetoaerialphotographsofyourcounty,youmightbeabletotellwhichwetlandsdryupatcertaintimesoftheyear.AGIS,whichcanuseinformationfrommanydifferentsourcesinmanydifferentforms,canhelpwithsuchanalyses.
Theprimaryrequirementforthesourcedataconsistsofknowingthelocationsforthevariables.Locationmaybeannotatedbyx,y,andzcoordinatesoflongitude,latitude,andelevation,orbyothergeocodesystemslikeZIPCodesorbyhighwaymilemarkers.AnyvariablethatcanbelocatedspatiallycanbefedintoaGIS.SeveralcomputerdatabasesthatcanbedirectlyenteredintoaGISarebeingproducedbygovernmentagenciesandnon-governmentorganizations.DifferentkindsofdatainmapformcanbeenteredintoaGIS.
AGIScanalsoconvertexistingdigitalinformation,whichmaynotyetbeinmapform,intoformsitcanrecognizeanduse.Forexample,digitalsatelliteimagesgeneratedthroughremotesensingcanbeanalyzedtoproduceamap-likelayerofdigitalinformationaboutvegetativecovers.AnotherfairlydevelopedresourcefornamingGISobjectsistheGettyThesaurusofGeographicNames(GTGN),whichisastructuredvocabularycontainingaround1,000,000namesandotherinformationaboutplaces.
Likewise,censusorhydrologictabulardatacanbeconvertedtomap-likeform,servingaslayersofthematicinformationinaGIS.
4.2Datarepresentation
GISdatarepresentsrealworldobjects(roads,landuse,elevation)withdigitaldata.Realworldobjectscanbedividedintotwoabstractions:
discreteobjects(ahouse)andcontinuousfields(rainfallamountorelevation).TherearetwobroadmethodsusedtostoredatainaGISforbothabstractions:
RasterandVector.
Rasterdatatypeconsistsofrowsandcolumnsofcellswhereineachcellisstoredasinglevalue.Mostoften,rasterdataareimages(rasterimages),butbesidesjustcolor,thevaluerecordedforeachcellmaybeadiscretevalue,suchaslanduse,acontinuousvalue,suchasrainfall,oranullvalueifnodataisavailable.Whilearastercellstoresasinglevalue,itcanbeextendedbyusingrasterbandstorepresentRGB(red,green,blue)colors,colormaps(amappingbetweenathematiccodeandRGBvalue),oranextendedattributetablewithonerowforeachuniquecellvalue.
Theresolutionoftherasterdatasetisitscellwidthingroundunits.Forexample,inaLIDARrasterimage,eachcellisapixelthatrepresentsanareaof3metersby3meters.Usuallycellsrepresentsquareareasoftheground,butothershapescanalsobeused.
Vectordatatypeusesgeometriessuchaspoints,lines(seriesofpointcoordinates),orpolygons,alsocalledareas(shapesboundedbylines),torepresentobjects.Examplesincludepropertyboundariesforahousingsubdivisionrepresentedaspolygonsandwelllocationsrepresentedaspoints.Vectorfeaturescanbemadetorespectspatialintegritythroughtheapplicationoftopologyrulessuchas‘polygonsmustnotoverlap’.
Vectordatacanalsobeusedtorepresentcontinuouslyvaryingphenomena.Contourlinesandtriangulatedirregularnetworks(TIN)areusedtorepresentelevationorothercontinuouslychangingvalues.TINsrecordvaluesatpointlocations,whichareconnectedbylinestoformanirregularmeshoftriangles.Thefaceofthetrianglesrepresentstheterrainsurface.
Thereareadvantagesanddisadvantagestousingarasterorvectordatamodeltorepresentreality.Rasterdatasetsrecordavalueforallpointsintheareacoveredwhichmayrequiremorestoragespacethanrepresentingdatainavectorformatthatcanstoredataonlywhereneeded.Rasterdataalsoallowseasyimplementationofoverlayoperations,whicharemoredifficultwithvectordata.Vectordatacanbedisplayedasvectorgraphicsusedontraditionalmaps,whereasrasterdatawillappearasanimagethatmayhaveablockyappearanceforobjectboundaries.
Additionalnon-spatialdatacanalsobestoredbesidesthespatialdatarepresentedbythecoordinatesofavectorgeometryorthepositionofarastercell.Invectordata,theadditionaldataareattributesoftheobject.Forexample,aforestinventorypolygonmayalsohaveanidentifiervalueandinformationabouttreespecies.Inrasterdatathecellvaluecanstoreattributeinformation,butitcanalsobeusedasanidentifierthatcanrelatetorecords
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