6/29/2015 IntegratingGPSDatawithGISData

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InDepthTutorialsandInformation

IntegratingGPSDatawithGISData

INWHICHyoulearnapproachestoconvertingGPSsoftware1datafilestoGISsoftwaredatafiles,^practicewithsampleGPSanddigitizedshapefilesandcoverages,andthenundertaketheprocessonyourown.

OVERVIEW

AnotherDatumLesson:IteachattheUniversityofKentuckysDepartmentofGeography,inLexington.SeveralyearsagoIborrowedaGPSreceiverfromTrimbleNavigationtouseinaclass.Ithadoccurredtomethatourdepartmentalfacultydirectorylistednames,officeandhomeaddresses,phonenumbers,andsoon,butnot,asseemedappropriateforgeographers,theirgeographiccoordinates.SoIsentmystudentsouttoparkinmycolleaguesdrivewaysandcollectGPSdata,whichweconvenedtoanArcInfocoveragebasedonWorldGeodeticSystemof1984(WGS84)UniversalTransverseMercator(UTM)coordinates.

WethendigitizedthemajorroadsfromLexingtonareaU.S.GeologicalSurvey(USGS)7.5minutetopographicquadmapsbasedontheNorthAmericanDatumof1927(NAD27).ThemapsindicatedthattoconvertNAD27toNAD83(whichiswithincentimetersofWGS84),oneshouldmovethegridlines4meterssouthand6meterswesthardlyanissueforus,becauseouraccuracywaslimitedbyselectiveavailabilitytoperhaps50meterswhencalculatedoveranumberofmeasurements.Itwas,therefore,distressingwhenagraphicoverlayofthesetwocoveragesputthecurrentdepartmentalchairshousesmackinthemedianofLexingtonslimitedaccessbeltway.(He,infact,livesacoupleoffootballfieldsnorthofthere.)

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WhatIlearned,theembarrassingway,wasthatwhiletheareaslatitudeandlongitudecoordinateswereadjustedbyafewmetersinthe1927to1983datumchange,theUTMcoordinateswereadjustedbymorethan200metersinanorthsouthdirection!Sothestatementonthemapthatthegridlinescanbemovedonlyafewmeterswasmisleadingitreferredonlytothelatitudeandlongitudegridlines.TheUTMgridandtheStatePlanegridhadmuchgreaterchanges.

ReviewingWhatYouKnow

Youhavebeengiven,oryouhavecollected,fileswiththeextensionsSSF(StandardStorageFormat)orCOR(fordifferentiallyCORrected)files.ThesefileshavebeenprocessedanddisplayedusingtheTrimblePathfinderOfficesoftware.YourgoalnowistousethesedatafilesinaGeographicInformationSystem(GIS)wheretheycanbeconsideredwithmanyotherdatasourcesthatyoumayhaveavailable.Thisisnotadifficultprocessbutyoureallymustbecareful:itisquiteeasytogetwhatlookslikeareasonableGISfile,butonethathasthelocationswrong,thusmakingtheactivityworsethanuseless.

ThekeytoconvertingfromTrimblefilestoESRIfilesistoknowthatyouneedonlyconvertfromSSF(orCOR)filestowhatiscalledanArcInfocoverageortoanArcViewshapefile.Onceacoverageorshapefilehasbeenobtained,theentirerangeofESRIproductsisavailableforyouruse.Itisalsotruethat,nomatterwhichTrimbleproductsyouuse,ultimatelyyouwillhavefilesintheirstandardfileformat.Soitisasthoughyouhavea"datatunnel,"withwiderangesofproductsoneachsidebutwiththerestrictionthatthedatamustflowthroughanarrowpassage(pleaseseeFigure61).

TheprocessofmakinganESRIcoveragebeginswiththePathfinderOfficesoftware.ThisGPSsoftwarewillgenerateasetoffiles.ThesefilesarenotESRIcoveragefiles,butratherarefilesthatArcInfocommandswillusetocreatethepropercoverage.Soyouwillbeexecutingamultistepprocess.TheresultultimatelywillbeanArcInfocoverage,whichisaDOS,Windows,orUNIXdirectory(akaafolder)containinganumberoffiles.

TheprocessofmakinganArcViewshapefileismoredirect.PathfinderOfficegeneratesashapefilefile(actually,threeormoreyouwillbeexecutingamultistepprocess.TheresultultimatelywillbeanArcInfocoverage,whichisaDOS,Windows,orUNIXdirectory(akaafolder)containinganumberoffiles.

6/29/2015 IntegratingGPSDatawithGISData

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Figure61.DataTunnel:TrimbleGPSdatatoESRIproducts.

TheprocessofmakinganArcViewshapefileismoredirect.PathfinderOfficegeneratesashapefilefile(actually,threeormorefilesinthesamedirectoryareneededtomakeanArcView"shapefile")thatarereaddirectlybyArcView.

ThereasontouseaGISwithGPSdataistocombinelocationaldatafromavarietyofsources.So,firstandforemost,youmustascertaintheparametersoftheexistingESRIcoveragesorshapefilesintowhichyouwishtointegratetheGPSdata.Ifyougetthiswrong,everythingwillbewrongthenceforth.Amongthethingsyou

6/29/2015 IntegratingGPSDatawithGISData

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mustconsiderare:

geodeticdatum(andworkinNorthAmericachoicesareusuallyNAD27,NAD83,WGS84)

projection,ifany,usedtoconvertthedatafromlatitudelongituderepresentationtoaCartesiancoordinatesystem,

unitsoflinearmeasure,(e.g.,meters,miles,surveyfeet,andmanymore),and

unitsofangularmeasure(almostalwaysdegrees,buttheissueofhowfractionalpartsofadegreearerepresentedcancomplicatethings).

PrescriptionforFailure:IncorrectParameters

IntheUnitedStates,thevaluesaredifferentinthenorthsouthdirectionabout200meters,dueprimarilytohumanslearningmoreabouttheshapeoftheirEarth.IntheWesternUnitedStatestheeastwestdifferencecanbearound100meters(e.g.,Seattlearea:93metersSanDiegoarea:79meters)itisusuallylessintheeasternpartofthecountry(e.g.,Bangor,MEarea:50metersMiamiarea:17metersLexington,KYarea:2meters).YouneedtobeconcernedthatyouconverttheGPSfiletothedatumusedbyanyshapefileorcoverageyouwishtocombinewiththeGPSdata.Youmaydeterminethedatumofthecoverageinavarietyofways.IfyourdataareinArcInfocoverageformattheremaybeanASCII"prj"(projection)filethatdescribestheparametersofthedataset.Ifyouareusingshapefilestheinformationmaybepresentaswellitwouldbefoundinafilewiththeextension"prj"appendedtothenameoftheshapefile.Aprojectionfilecontainsinformationsuchas:

Projection STATEPLANE

Zone 3976

Datum NAD83

Zunits NO

Units FEET3

Spheroid GRS1980

Xshift 0.00000000000

Yshift 0.00000000000

Parameters

Youshouldprobablycarefullyinvestigatethesourcesofthedataandtheirprocessinghistorysoyoucanbe

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certainofthedatumandotherprojectionparametersused.

Projection:Asyouwillrecall,the2DcomponentsoftheTrimbleSSFandCORfilesrepresentdatainthelatitudeandlongitudedatumofWGS84,thoughtheymaybedisplayedinotherforms.Afundamentaldilemmaofaspatialanalystisthatthemostaccuratewaytodepictapointontheearthssurfaceiswithlatitudeandlongitude,butthenumbersthatrepresentsuchapointareinacoordinatesystem(spherical)thatmakesithardertousethesenumbersincalculationforsuchquantitiesasdistanceanddirection.Makingthesecalculationsinaprojectioniseasier,but,ofcourse,yougeta(usually,slightly,ifyouarecareful)wronganswer.

Further,mapsthatcoveralotofareathatareshowninlatitudelongitudeare(usually)badlydistortedvisually,somostGISuserselecttostoredatainsomeprojectioninwhichthehorizontalandverticaldistancesonthemapcorrespondtotheeastwestandnorthsouthdistancesontheEarthssurface.Thesemapsappear(generally)muchlessdistorted.However,thereisnowactualmathematicaldistortionforallbutafewpoints.Thereisreallynogoodsolutiontothisdilemmayoumustacceptsomeinaccuracy.

Whatsvital,however,isthatyoutelltheTrimbleconversionprocessthecorrectprojectiontousesothattheinaccuraciesinyourGPSdatawillbeconsistentwiththoseintheotherdatayourareworkingwith.ArcView3.xallowsyoutodisplaydatainanothercoordinatesystembesidestheoneinwhichitisrecorded.Further,ArcView3.2containsroutineswhichallowyoutoconvertoriginaldatafromoneprojectiontoanother.

LinearMeasurementUnits:Thechoicesaremeters,feet(international),andsurveyfeet.SurveyfeetformedthebasisoftheNAD27datum4internationalfeetwereusedinNAD83andWGS84.Whatsthedifference?Notmuch,butenoughtobesignificantinsomesituations.ThedifferencescomefromaslightdistortionofeachEnglishunittomakeitconformtothemetricsystem.Aninternationalfootisbasedontheideathatthereareexactly0.0254metersinaninch.Asurveyfootisbasedontheequalityofexactly39.37inchesandameter.Ifthesetwoconversionswereequivalent,youshouldgetexactlythepure,unitlessnumberone(1.0000000)whenyoumultiplythem(0.0254metersperinchtimes39.37inchespermeter).Theyarentandyouwont.Whatistheproduct?Useacalculator.

Thefractionalpartmaylooklikeaninsignificantnumber,butitcanmeanamatterofseveralfeetacrossaStatePlaneCoordinatezone.

AngularMeasurementFormat:Angularmeasurementunitsareimportantonlyifyouareconvertingafiletoacoveragewhichusesthelatlongraticuledirectly.Ifyoudousethegraticule,youwillwanttoselectdegrees,anddecimalfractionsthereof,becauseESRIproductsdontdirectlyutilizeminutesandsecondsascoordinatevalues.

TheOldConundrum:the"Spherical"EarthandtheFlatMap

TheGPSdatainthereceiverandinSSFfilesarestoredinlatitude,longitude,andheightaboveellipsoidcoordinates.Youcan,ofcourse,makeESRIshapefilesorcoverageswithlatitudeandlongitudedirectly,aslongasyouselectdegreesandfractionsofadegreeastheoutputnumbers,beingsuretouseenoughdecimalplaces.Youmaywanttodothisifyouaregoingtocombinethedatawithothercoveragesthatarestoredinthat"projection."Afterall,thisisthemostfundamental,accurateway.Youmustrealize,however,thatanygraphicrepresentationofthesedatawillbebadlydistorted,exceptneartheequatorwhereadegreeoflongitudecoversapproximatelythesamedistanceasadegreeoflatitude.Anywhereelse,anyimageofthecoverageisvisuallydistorted,andthelengthsofmostlinesisvirtuallymeaningless,sincethe"length"isbasedondifferencesbetweenlatitudeandlongitudenumbers.SuchnumbersdonotprovideaCartesiantwodimensionalspace.(Recalltheoldriddle:wherecanyouwalksouthonemile,eastonemile,andnorthonemile,onlytofindyourselfbackatthestartingpoint?5NotonanyCartesianxygrid,forsure.Descartes,forhisplane,insistedthataunitdistanceinthe"x"directionbeequivalenttoaunitdistanceinthe"y"direction.)

6/29/2015 IntegratingGPSDatawithGISData

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