6/29/2015 BeyondtheBasics(GPSandGIS)Part1

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BeyondtheBasics(GPSandGIS)Part1

INWHICHyoulearn(a)howtocollectattributeinformationabouttheenvironmentsimultaneouslywithcollectingGPSpositiondata,(b)howtoplanforimportantGPSmissions,(c)aboutdifferentialcorrectionthattakesplaceimmediately,and(d)howtonavigateusingaGPSreceiver.1

OVERVIEW

Attributes,GPS,andGIS

Overheard:GlobalPositioningSystem?Asystemforpositioningtheglobe?

ObtainingGISAttributeDatawithGPSEquipmentandSoftware

Fromasoftwarepointofview,aGIScouldbedefinedasthemarriageofagraphic(orgeographic)database(aGDB)withotherdatabasesmostfrequentlyarelationaldatabase(RDB).TheseotherdatabaseswhichcontainattributedataaboutfeaturesintheGDBareusuallytextualinnature,butsometimesconsistofdrawings,imagesorevensounds.(Forexample,youcouldkeyin,orclickon,astreetaddresstoyourGISandbeshownaphotoofthehousethere.)ThecombinationofaGDBandRDBallowstheusertomaketextualqueriesandgetgraphicalresponses(e.g.,showwithared"X"thosestreetlightswhichhavenotbeenservicedsinceAugust1995)or,conversely,makeagraphicalqueryandgetaresponseintext(e.g.,indicatethedailyyieldfromparkingmetersinthisareathatIhaveoutlinedonthisimageofthecityusingamousepointer).

IfaGISisadatabasewithattributeinformationaboutgeographicalfeatures,thenitseemsreasonabletocollecttheattributedataatthesametimethepositionaldataarecollected.ThusfarinthistextwehavenotdonethiswithGPS.TheprocessofaddingattributedataoccurredafterwegeneratedtheGIScoverages,sinceourGPSfilescontainedonlypointsin3Dspace.Supposeyoucollectedasequenceoffixesalongatwolaneroad.OncethearcwasdepictedintheGIS,youwouldhavetolateraddthe"twolane"facttotherecordwhichrelatedtothearc,ifyouwantedthatinformationinthedatabase.

ProbablythemostefficientandaccuratewaytouseGPStodevelopaGISdatabaseistocollectthepositiondataandtheattributedataatthesametime.SinceahumanoperatorisrequiredtotakethepositiondatawithaGPSreceiver,itmakessensetohaveherorhimentertheattributedataaswell.SomeGPSreceivers,includingtheGeoExplorers,allowthissortofdatacollection.

TheOrganizationofAttributeData

TheentryofattributeinformationintoaGISbyusingaGPSreceiverisfacilitatedbyadatadictionary,whichisahierarchicalcollectionoftextualtermsstoredintheGPSreceiversmemory.Thetermsfallintothreecategories:

Feature

Attribute

Value

"Feature"isusedasitisinArcInfoandArcView.Itreferstothefeaturetypethatisthesubjectofacoverageorshapefileitisthe"Feature"of"FeatureAttributeTable"suchasanArcInfo"PAT"(PointAttributeTable)or"AAT"(ArcAttributeTable).InArcView,theparallelconceptofFeatureistheTheme.AfeaturetypebecomesanArcInfocoverageoranArcViewtheme(madefromashapefile).Examplesoffeaturetypesareparkingmeters,paved

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roads,andlanduse.Apoint(ortheaverageofasetofpoints)collectedbythereceiverforagivenfeaturemaybecomethebasisforarecordinaPATorarecordinanArcViewpointthemetable.AsequenceofpointsrecordedalonganarcmaybecomethebasisforarecordinanAAT,orarecordinanArcViewlinethemetable.

"Attribute"isalsoaconceptparalleltooneinArcInfoandArcView.Attributesarethe"items"or"columns"or"fields"ofatable.Suppose,forexample,youaredevelopingaGISdatabaseaboutparkingmeters.Inadditiontothepositionaldatacollectedatthemetersautomatically,youmightwishtobeabletorecordeachmetersidentifyingnumberasoneattribute,andtheconditionofthemeterasasecondattribute.Informationaboutthemonthlyrevenuecouldbeaddedlaterasathirdattribute.

"Value"referstotheactualnumericorcharacterentriesinthetable.Continuingourparkingmeterexample,youwouldenterameternumberastheidentifierofaparticularparkingmeterandselectamenuitem"fair"toindicatetheconditionoftheunit.

Asyoucansee,thereisahierarchytotheseterms:afeaturecontainsattributesattributesarecolumnsofvalues.

OnceyouhavecollectedfeaturedatawithaGPSreceiver,ifyouareconvertingthesedatatoArcInfocoveragesorArcViewthemes,eachfeaturetype2becomesaseparateArcInfocoverageorArcViewshapefilewithitsownfeatureattributetable.Thetableconsistsoftheusualinitialitems(e.g.,inArcInfo:area,perimeter,internal,anduser_idsforaPATfromandtonodes,leftandrightpolygons,length,interval,anduser_idsforanAAT)pluscolumnsforeachattributeforwhichtherearedata.Ifagivenfeature(record)hasadatavalueforagivenattribute,thatvaluebecomesanentryinthefeatureattributetableorthemetable.

TheDataDictionary

Adatadictionaryneednotbelongorcomplicated.Forexample,considerthefollowingone,namedVery_Simple.Itcontainsonlyfeaturetypesitcontainsnoattributes,soalsonovalues.

Rocks(point)

Trees(point)

YoucouldmakethisdatadictionaryonyourPCusingthePathfinderOfficesoftware.ThenyoucouldtransferittoyourGeoExplorerreceiver.

TocollectfeaturedatawiththeVery_Simpledatadictionary,youmightbeginaGPSfile(forexample,A010101A),settingupthereceivertorecordafeaturedatapointevery10seconds.Youmightthenmovearoundtheareaofinterest.Whenyouarrivedatatree,youwouldselectthefeature"Tree"fromtheGeoExplorermenuandcollectanumberoffixesat,say,10secondintervals.Youwouldthenclosethefeatureand,perhaps,movetothesiteofanothertree,openingthefeature,collectingdata,andclosingthefeature.Shouldyouencounterarockyoucouldopenthefeature"Rock"andrecordfixesthere.Thefixesyourecordateachindividualobjectareautomaticallyaveragedtoproduceasinglepointthatapproximatesthepositionoftheobject.

UponreturningtoyourPC,youwouldusePathfinderOfficetotransferthefileA010101A,andgeneratePOSNPTS.SSF.

ThenyoucouldusePOSNPTS.SSFtogeneratetheappropriateESRIfiles.IfyouwantArcViewshapefilesyoucangeneratethesedirectlywiththePathfinderOfficeSoftware.

POSNPTS.SHP

ROCKS.SHP

TREES.SHP

POSNPTS.SHPwouldconsistofthepointsthatthereceiverrecordedwhileyouwerewalkingbetweenfeatures.

ROCKS.SHPwouldcontainasinglepointforeachrockyouvisited.Thepointforagivenrockwouldbetheaverageofthefixescollectedatthatrock.Thatpointwould,therefore,provideapproximatecoordinatesforthatrock.APATwouldbeformedwithseveralrecords,onerecordforeachrock.(ThesameideawouldholdtrueforTREES.SHP.)

FeatureswithAttributesAttached

Youmightinsteaduseasomewhatmorecomplexdatadictionary,Still_Simple,whichmightlooksomethinglikethis:

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Assumingdataweretakenforallthefeaturetypes(Rocks,Trees,Streets,Intersections,Hydrants,andStreetlights)sevenshapefilescoverageswouldresult:POSNLINE.SHP,ROCKS.SHP,TREES.SHP,STREETS.SHP,andINTERSEC.SHP,HYDRANTS.SHP,andSTREETLI.SHP.(Generatedshapefilenameswillbeamaximumof8characterslong).

Inadditiontothestandarditems(columns,fields)intheshapefiletable,ROCKS.SHPwouldcontainthecolumnsSIZEandCOLOR.Thevalueswhichcouldappearinthesecolumnswouldbe"Small,""Medium,"and"Large,"ornovalueatallforSIZE.WhatvaluescouldappearintheCOLORcolumn?

FromtheEnvironment,throughGPS,toGIS

TheprocessofrecordingattributedatawithaGPSreceiverisagoodbitmorecomplexthansimplyrecordingpositiondata,whichisitself,asyouknowonlytoowell,notatrivialmatter.Torecordattributedatayouhavetogothroughseveralsteps:

BuildaDataDictionarywithacomputer.ThiscanbedoneonaPCusingthePathfinderOfficesoftware.

LoadthedatadictionaryfileintotheGeoExplorer.ThisprocessissimilartotransferringpositionfilesandalmanacsfromthereceivertothePCthedatasimplygotheotherdirection.ThePCmaycontainanumberofdatadictionariesaGeoExplorermaystoreonlyoneatatime.(AGeo3maycontainmultipledatadictionaries.)

Takethereceivertothefieldandopenafile.Selectaparticularfeaturetype(forexample,apointfeature)fromthemenu.Whiletheunitisautomaticallycollectingpositioninformation,youmanuallyselecttheappropriateattributeandvalueitems.Whenenoughfixeshavebeencollected,stopthedatacollectionprocessforthatfeature(i.e.,closethefeature).Ifthefeaturetypeisofthe"point"type,thefixesobtainedwillbeaveragedsothatasinglepointrepresentsthegivenpointfeature.

Continuetocollectdatainthefield.ThedatacollectedwhennofeatureisselectedmayultimatelybecomeoneArcInfocoverageorArcViewshapefile.Foreachfeaturetypeforwhichyoucollectdata,acoverageorshapefilemaybebuilt.Thatcoverageorshapefilewillcontainthenumberofindividualfeaturesforwhichyourecordedpositioninformation.NotethatagivendatacollectionsessioncouldresultinanumberofArcViewshapefiles(orArcInfocoverages),thedataforwhichmightallbecontainedwithinoneGPSfile.Linearfeaturesmaybecollectedinasomewhatsimilarway.

ClosetheGPSfile.Openanewoneifyoulikeandcollectadditionalfeaturedata,orsimplypositionaldata,asyouwish.

ReturntoyourofficeorlabanduploadthefilesfromyourGPSreceivertothePCintheusualway.

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Differentiallycorrectthefiles.

UsingPathfinderOffice,produceArcViewshapefiles,whichwillbecomethemes,orproducetheSMLs(forPCArcInfo)orAMLs(forworkstationorUNIXArcInfo)andassociatedESRIfilesthat,whenexecuted,willproduceArcInfocoverages.

TopracticeseeingandusingthefeatureattributecollectionabilitiesofGPS,doPROJECTS7A,7B,7C,and7D.

STEPBYSTEPAttributes,GPS,andGISPROJECT7A

DemonstratingFeatureAttributeData

InMarchof1999ondifferentoccasions,alightplanewasflownfromLexingtontosurroundingairportsrecordingGPSpositionandfeaturedata.Usuallya"touchandgo"(brieflandingandtakeoff)wasaccomplishedinwhichtheaircraftwheelscontactedtherunwayandaGPSfix(includingtheGPSaltitudeandlocaltime)wasrecordedintheGPSreceiver.Featuredata,includingthealtitudereadingfromtheaircraftaltimeterandthepublishedaltitudeoftheairport,waslikewiseinput.Youwillexamineoneoftheseflights:atripfromBluegrassAirportnearLexington,KentuckytotheMontgomeryCountyAirportnearMountSterling,Kentucky.Onthisparticularflightafullstoplandingwasdoneatthedestinationairport.ThedataloggingwasdonewithaTrimbleAspenPCMCIAcard(withrequiredexternalantenna)inalaptopcomputer,ratherthanaGeoExplorer.

TheAspensystemhasseveraladvantagesoversimplyusingaselfcontainedreceiverdatalogger.Forone,youhaveafullkeyboardwithwhichtoenterdata.

Figure71.Screenlayoutofmap,timeline,position,andfeaturewindows.

Further,youhaveascreenwithwhichyoucandisplayamapoftheareaandseeyourlocationsandprogressincontext.Ontheotherhand,thelaptopislessruggedandusesmorepowerthanaGeoExplorer,andsomelaptopscreensaredifficulttoreadinbrightsunlight.ThecostofAspensoftwareandPCcardislessthanaGeoExplorer.Whenyouaddthecostofthelaptop,thetotalcostismore.Butyoumayalreadyhavethelaptopcomputer.Asyoucansee,therearemanyconsiderationsinvolvedinwhatGPSdataloggingsystemyoushoulduseforparticularapplications.

{__}InPathfinderOfficesetupthefollowingparameters:Kentuckystateplanecoordinates,northzone,NAD83surveyfeetforalllinearmeasurementstimezoneESTaltitudeMSL.

{__}OpenuptheMap,TimeLine,PositionProperties,andFeaturePropertieswindows.MaketheMapwindowoccupymostoftheupperpartofthelefttwothirdsofthescreenandarrangetheotherwindowsasshowninFigure71.

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{__}Inthefolder_:\GPS2GIS\Flying_Tour,openthefileR032720A.CORandthebackgroundgeotifffilesLEX.TIFandMTS.TIF.RepresentNotInFeaturedatawithmediumreddots.ZoomtoExtents.

{__}ZoomupthewesternmostimagesothatitfillstheMapwindow.YoushouldseemostofBluegrassAirport,includingthenortheastend(thedepartureend)ofrunway4.(SeeFigure72.)TheGPStrackwasstartedjustaftertakeoff.UsingPositionProperties,determinethealtitudeoftheplaneatthefirstGPSfix._.Thepublishedairportelevationis979feet.

Airportrunwaysaredesignatedbytakingtheirapproximatemagneticheading(tothenearest10)anddroppingthefinalzero.Soaircrafttakingofforlandingonrunway4headapproximately40,thatis,almostdirectlynortheast.

{__}Zoomuponthenortheastendoftherunway.Youcanseenumberspaintedontheconcrete.Aircraftlandingtowardortakingofftothesouthwestwouldrefertothisrunwayas"twotwo"220isthereciprocalof40.UseZoomtoPrevioussothattheentireairportimagefillsthemapwindow

{_}InView~Layers~FeaturesyouwillnoticefeaturenamesofAltitudeandAirport.Represent"Altitude"withabluedotand"Airport"withagreendot.PathfinderOfficeisawarethattheAltitudeandAirportlayersexistbecauseyouhaveopenedafilethatcontainsthosefeaturetypes.Represent"NotInFeature"dataasfatreddots.

{__}MaketheMapwindowactivebyclickingonitstitlebar.TurnonAutopantoSelection.Usingthepositionpropertieswindow,push"First"andthenbeginmovingalongtheGPStrackwiththe">"button(orAlt>keycombination).Notetheaircraftsaltitudesduringtheclimboutphaseoftheflight.Atpoint14or15,stopandexaminethetrack.Youshouldseeabluedottotheeastoftheselectedfix.

Thebluedotrepresentsaquickmarkapointfeaturethatismadewhilemovingalongapath.Aquickmarkspositionconsistsofaninterpolationbetweenthetwofixesthatboundit,intermsoftime,onebeforeandoneafter.Normally,asyouwillseelater,apointfeaturespositionismadeupoftheaverageofanumberoffixesthatarecollectedatthesamespot.Youcantdothiswhilewhizzingpastapointofinterestat100milesperhour,sosomeGPSequipment,suchastheAspenPCMCIAcardwithalaptopcomputer,allowsyoutocollectquickmarks.Aquickmarkidentifiesapositionandtheoperatorcanputintheattributeinformationlater.(WithotherGPSequipmentwithoutthequickmarkfeatureyousimplyhavetotakeapointfeatureconsistingofasinglepointattherightmoment.)

Figure72.GPStrackdepartingBluegrassAirport.

{_}ContinuemovingalongtheGPStrack.Afterfix16,"NotinFeature:16"willchangeto"Position:Quickmark."IntheFeaturePropertieswindowyouwillsee"MSL0"and"Barometer29.92."3Thesearedefaultvaluesforthepointfeaturethatis,theoperatortriggeredthequickmarkthatrecordedthepositionbutdidntenteranyfeaturevalues.

{__}ContinuealongtheGPStrack.NoticethatthecountingofNotinFeaturefixeshasrestartedat1.Thereareonlythreesuchfixesbeforethenextquickmarkpointfeature.Moveaheadtothatpointfeature.Heretheoperatorrecordedthattheaircraftaltitude(MSL)was2,800feet,accordingtothealtimeter,whichwassetat30.18thevaluethatthegroundcontrollershadtransmittedtothepilotbeforetakeoff.WhatwastheGPSaltitude?

{__}ContinuealongtheGPStrackwiththe">"button.Youmaynotethattherearesome60plusfixesremainingandthatthesceneryhasbecomeverydull.Wecanspeedthingsup.MaketheFeaturePropertieswindowactivebyclickingonitstitlebar.Ifyoupausethepointeroverthe">"buttoninthatwindowyouwillseethatitwilltakeyoutothenextfeature.Clickthatbutton.

{_}YoushouldseeaDOQoftheairportnearMountSterling.

Justastheplanelandedaquickmarkwastaken.ExaminetheinformationintheFeaturePropertieswindow.

TheAirportNamewasselectedfromamenuinthedatadictionary(weknewwhichairportswewouldbeflyingtoIOBistheairportdesignation).Thealtitudefromthealtimeterwasreaddirectlyfromtheinstrumentandtypedintothelaptopcomputer.Thetimewaslocaltime,basedonUTCtimetakenautomaticallyfromthereceiver.Thepublishedaltitude,therunwaynumber,4andtherunwaylengthwereinputafterconsultinganaeronauticalchart.Notice

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thedifferencesbetweenthealtimeteraltitude(1,040feet),thepublishedaltitude(1,020feet),andtheGPSaltitude(takenfromthePositionPropertieswindow)of1,012.644feet.ThedifferencesinthesealtitudesindicatewhyonedoesntwanttouseanaltimeteraltitudeoraGPSaltitude(evenacorrectedone)asthesoledeterminerofaltitudewhengettinganaircraftontheground.Amatteroftwoorthreefeetverticallycanmakethedifferencebetweenasmoothlandingandacontrolledcrash.

{__}YoucanexperimentbyclickingonvariousiconsintheTimeLinewindow,intheFeaturePropertieswindow,inthePositionPropertieswindowandontheMapwindow.Sometimesthesewindowsdontseemtobecompletelycoordinated,butyoucanalwaysultimatelygettoviewagivenentityinseveralways.Practicesteppingthroughthevariousfeaturesfromthefirsttothelast.

{__}IntheUtilitiesmenu(oronthe(UtilityToolbar)selectDataDictionaryEditor~File~Open.Navigateto__:\GPS2GIS\Flying_Tour\Apt_Info.ddfandopenit.Thisisthedatadictionaryfile(ddf)thatwasusedtocreatethefeaturesinR032720A.COR.ClickonthevariousFeaturesandAttributestobetterunderstandtherelationshipsbetweenfeatures,attributes,andvalues.Illsaymuchmoreaboutdatadictionariesandtheiruseinthefollowingprojects,butthisexerciseshouldgiveyouthegeneralideaofhowitallfitstogether.

Nextpost:BeyondtheBasics(GPSandGIS)Part2

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