GPS and GIS

6/29/2015 BasicConcepts(GPSandGIS)Part4

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BasicConcepts(GPSandGIS)Part4

PreparingtoCorrelateGPSDatawithMapData

{_}TakeouttheUSGStopographicquadrangle(atopomap,usualscale1:24,000)oftheappropriategeneralarea.Thereceivercandisplayageographicpositioninseveralcoordinatesystems.Under"Configuration~Coordinates"youwillfind

Degrees,Minutes,anddecimalfractionsofminutes(Deg&Min)

Degrees,Minutes,Seconds,anddecimalfractionsofseconds(Deg,Min&Sec)

OrdinanceSurveyofGreatBritain(OSGB)

UniversalTransverseMercator(UTM)

EarthCentered,EarthFixed(ECEF)

TrimbleGrid

{__}Choose"Deg&Min."

{__}Under"Configuration~Datum"youwillfindalistwithalargenumberofchoices,storedinalphabeticalorder.Youcanscrollthroughthelistbyholdingdowneitherthe"up"keyorthe"down"key.ChooseBahamas18(NAD27)withthe"CMD"key.

{__}ThenchooseWGS84(World),whichisthefundamentalGPSdatum.WGS84identifiestheWorldGeodeticSystemdevelopedin1984.

{_}Finally,notethedatumofthemapyouareusing.Thedatumisusuallyfoundinthelowerlefthandcorner(e.g.,NAD27,theNorthAmericanDatumof1927).Under"Configuration~Datum"settheGPSreceivertooperateinthisdatum,shownas"NAm.1927Conus"ifthisisthedatumofyourmapasitiswithmanyUSGStopographicalmaps.

PositionMeasuringSystem

EasyInstall,CompactSize,LongLifeHighfunctionandperformance!

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TosettheseoptionswiththeGeo3youuseFn&OPTIONSetup~Configurations~Coordinates.TheGeo3containsanannoyingnumberofDatumchoices.Ifyouholddownanarrowkeyitscrollsthroughthemnotquickly,butitbeatswearingoutyourthumbandtheswitch.Ashortcut:thedatumchoicesareinalphabeticalorderthelistwrapsaroundtheend(i.e.,"A"comesafter"Z").

TheotheroptionsyouneedtosetinthissectionareunderFn&OPTIONSetup~Configurations~Units.

CustomSetup

1.DistanceUnits

2.AngularUnits

3.VelocityUnits

4.Altitude

5.AltReference

6.NorthReference

{_}Under"Configuration~Units~CustomSetup"youwillfindthismenu:

{_}Under"DistanceUnits"chooseKilometers.DistanceunitsavailableontheGeoExplorerincludeYards,Meters,Kilometers,NauticalMiles(6080feet),Miles(statute,5280feet),"InternatlFeet"(internationalfeet,whereaninchis0.0254meters,exactly),and"U.S.SurveyFeet"(whereameterisconsideredtobe39.37inches,exactly).

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{_}Examineyourmaptodeterminetheappropriatedistanceunitsfortheupcomingfieldwork.Feelfreetochangethisvaluelaterifyoushouldwishvisualoutputinsomeotherunits.

{_}Under"Configuration~Units~CustomSetup~AltitudeUnits"chooseMetersorFeet,dependingonthemapyouwillbeusing.

{_}Under"Configuration~Units~CustomSetup~Altitude

Reference"chooseGeoid(MSL).The"altitudereference"maybesettoeitherMSL(whichiselevationaboveMeanSeaLevel)orHAE(whichisHeightAbovethereferenceEllipsoidthetheoreticalmathematicalsurfacethatapproximatesthesurfaceoftheEarth).SinceatthistimeyouprobablydonotknowtherelationshipbetweentheHAEandMSLatyourlocation,youareselectingMSL.

{__}Under"Configuration~Units~CustomSetup~NorthReference"choose"North,True"or"South,True"dependingonthemapyouwillbeusing.

DoubleCheckingtheConfiguration

{__}Nowdoasummarycheckoftheconfiguration.Underthemenuchoice"Main~Configuration"checkthefollowingcriticaloptions:

RoverOptions~Dynamics:Land

RoverOptions~PosMode:3D

RoverOptions~ElevMask:15

RoverOptions~SNRMask:4

RoverOptions~PDOPMask:6

RoverOptions~AntennaHt:1.00(oryourwaistheightinmeters)

RoverOptions~LogDOPs:Off

RoverOptions~Velocity:Off

RoverOptions~FilePrefix:(donotchange)

RoverOptions~NotinFeatureRate:Off

Coordinates~Deg&Minutes(anddecimalfractionalpartsthereof)

Datum:(settoyourmap)

Units~Custom~Distance:(settoyourmap)

Units~Custom~Angular:Degrees

Units~Custom~Velocity:KilometersperHour

Units~Custom~AltitudeUnits:(settoyourmap)

Units~Custom~AltitudeReference:Geoid(MSL)

Date&Time~SetLocalTime:(adjusttolocaltime)



Date&Time~TimeDisp:Local12Hours

BatteryUsage:(resetifbatteryfreshlycharged)

UsethischecklisteachtimeyoutaketheGeoExplorerintothefieldmodifyingitasnecessary.

FinalInsideActivity

YouarealmostreadytotaketheGeoExplorerintothefield.Onethingremainstobedone.Whileyouarestillinside,readthroughthedirectionsforPROJECT1Bbelowcompletelytoprepareyourselfforthefieldwork.Developafeelforthesortofdatayouwillbecollecting.Practicechangingfromscreentoscreen.Outside,withthewindblowingandthetrafficroaring,isnotimetodiscoverthatyoudonthaveasolidsurfacetowriteonorthatyoudontknowjustwhatitisyouaresupposedtobedoing.Alittlepreparationnowwillpaybigdividendslater.

IfyouaregoingtocollectdatawiththeGeo3youalsoneedasquareabout2.52.5inchesmadeofacoupleoflayersofaluminumfoil.Andafewinchesofmaskingtape.

{__}ReadoverPROJECT1Bbelow.

PROJECT1B

NowOutside

Thisisanexercisebestdonewithtwopeople.Youwilltakethemap,yournotebook,andtheGPSreceiveroutsidetomakeobservations.Youwillnotyetplacethedatayoucollectintoacomputerfilebutyouwilllearnalotaboutthefactorsaffectingdatacollection.(IfyouarenotsurethatthesettingsonthereceiverarethoseyouputinduringProject1A,verifythemagainstthoseintheprevioussection:"DoubleCheckingtheConfiguration.")

{__}Asyouleavetheclassroomorlaboratorytotraveltothesitefordatacollection,turntheuniton.Ifyoucarrythereceiverexposedtothesky,itwillbeginto"acquire"satellites.Itisreofimportantwhichmenuappearsonthedisplaywheneverthereceiverison,it"looks"forsatellitesandcalculatespositionsifitcan.

{__}Movetoaspotoutdoors,wellawayfrombuildingsandheavytreecanopy.Ifitisreasonablylevelandnotshroudedbynearbyhillsormountains,somuchthebetter.Andifyoucanlocatetheantennaoverageodeticmonument,forwhichyoucanfindtheofficiallatitudeandlongitude(perhapsfromtheNGSwebsite,www.ngs.noaa.gov),super.

{__}Lookatthemaptolocateyourapproximateposition.

{__}Holdtheantennaoverthespotforwhichthecoordinatesaretobedetermined.Theantennainthereceiverisjustbelowtheimageofthesextantembossedintheplasticofthetopofthereceiver.Holdtheunitasfarinfrontofyouasiscomfortable,withthetoppartclosetohorizontal,tiltedonlyenoughsoyoucanreadthescreen.

GPSStatus

I.SatTracking



2.SatPosn&SNR

3.SatHlth&URA

4.SatChnlData

Actually,nopositionwillbecomfortableafterafewminutesyouwillwanttopasstheunittoyourpartnersoyoucandropyourarmandlettheblooddrainbackintoyourfingers.Analternativeistoputtheunitonthegroundandcrouchorsitdownsoyoucanreadthescreen.Thisislessfuninwinter,orwhenthereispoisonivyabout.Ineverclaimedfieldworkwaseasy.Youmightbringatableortripodwithyou,orfindafencepost.Becareful:thepowercordmakesiteasytobouncethereceiverofftheground.Itsatoughunitbutitisalsoexpensivedoyoureallywanttotestit?

{_}Keepyourheadandbodyoutoftheway,i.e.,dontblockthesignalfromasatellitetothereceiver.Youareopaque,asfarasthehighfrequency,shortlengthGPSwavesareconcerned.Remember,thereceiverislookingforsatellitesaslowas15degreesabovethehorizon.Itseasytoforgetthisandobstructtheantenna,causingthereceivertoloseitslockonasatellite.

TrackingSatellites

{__}StartingwiththeMainMenu,navigatetothe"GPSStatus"menuandpress"CMD."Thefollowingwillappear:Highlight"SatelliteTracking"andpress"CMD."The"SatTracking"screenwillappear,listingsometwodigitnumbers.Thesearethedesignations,calledPRNnumbers,19thatyourGPSreceiverusestoidentifythesatellites.Thenumbersliebetween1and32,inclusive.

Thenumbersthatappearnowarethoseofthesatellitesthatthereceivermightbeabletopickup,basedonyourpositionandtime.Theyareusuallythosewhichareabovethehorizonandthespecifiedelevationmaskangle.Thereceiverdetermineswhichsatellitesareavailablebyformulasbuiltintoitscomputerandbyanalmanactransmittedbyeachsatellitewhichdescribesthegenerallocationofallthesatellites.

Sinceyouareoutside,presumablythereceiverislockedontosomesatellites.Thenumberoflittleshadedboxesinthelowerlefthandcornerofthescreenindicateshowmany.Thereceiverneedstobereceivingatleastfoursatellitesbeforelocationfixesarecomputed.20Becausethegeometrywhentheelevationangleissetat15allowsthereceivertoconsidersatellitesinaboutonethirdofthesky,itmaytrackeightsatellites,orevenone



ortwomore.

Onemorebitofinformationmaybelearnedfromthisscreen.Smallarrowsuptofourofthemmaybeseenpointingtosatellitenumbers.Thesearethesatelliteswhichthereceiverisusing(orattemptingtouse)tocalculateitsposition.Tosummarizethescreen:

Ifasatellitesnumberappearsonthescreen,then,accordingtothealmanacthesatelliteshouldbephysicallyinthespaceabovetheuser,atananglegreaterthanthesettingoftheelevationmask.Thiswouldtheoreticallymakethesatelliteavailableforpositionfinding.

Ifanarrowappearsnexttothenumber,thesatelliteisbeingconsideredforpositionfinding.Ifapositionisfound,thesatelliteisbeingusedforpositionfinding.

Thenumberoflittleboxesindicatesthenumberofsatellitesthatthereceiveristracking,thatis,electronicallylockedonto.

TheGeo3displayismuchdifferentactuallymucheasier.GoFn&OPTION~GPS.ByusingtheOPTIONbuttonyoucantogglebetweentheStandardscreenandtheAdvancedscreen.TheStandardscreenshowsaskyplotofthesatellitesthatarebeingusedforpositionfinding,thatarebeingtracked,andthatcouldbetracked.Italso,mostimportantly,showsthecoordinatesoftheantennaofthereceiver.TheAdvancedscreenshowsatableofsatelliteinformation.Acheckmarkindicatesthesatellitecouldbeorisbeingusedtocomputepositions.

{_}Afewminutesmayelapsebeforetheunitlocksontoenoughsatellitestobegingivingpositionfixes.Ifmorethan10minutesgobywithnopositionfix,changethePDOPtoeight(8)andmakesureyouarentobstructingthesignal.

{_}OncetheGeoExploreristrackingfourormoresatellites,select"Position"fromtheMainMenuandwritedownthelatitude,longitude,andaltitude.Whenlockedontofourormoresatellites,thereceivercomputesthepositionoftheantennaaboutthreetimeseverytwoseconds.(Iftheword"OLD"appearsonthescreenitindicatesthatthevaluepresentedisonethatwascollectedinthepastperhapstheimmediatepastandthatthereceiverisnotcalculatingnewpositions.Makecertainthattherearenoobstructionsblockingthesignals.)

{_}Notethetime.Plantowritedownanewpositionreadinginyournotebookeveryminute,approximatelyontheminute,forthenextquarterofanhour.

{_}Inbetweenwritingpositionfixesinyournotebookyoushouldrecordsomeotherinformation.Movebacktothe"SatTracking"screen.Notedownthenumbersofthesatelliteswhichappearthere.Circlethenumbersofthesatellitesthereceiverisusingtocomputepositions.Alsonotehowmanysatellitestheunitisreceivingsignalsfrom.Writedownthevalueidentifiedas"PDOP."

{_}Nowitisprobablyabouttimetogobacktothe"Position"screentowritedownthenextsetofpositioncoordinates.Theyshouldbecloseto,butnotexactlythesameas,thoseyouwrotedownaminuteago.Thescreenshouldnotsay"OldPosition."Ifitdoes,youprobablygotyourheadinthewayofasatellitesignal.

{__}Nowgotothe"SatPosn&SNR"screen(itsunder"GPSStatus").Youwillseeseveralhorizontallinesofinformationoneforeachsatellitebeingtracked.Oneitemofinformationdisplayedforeachsatelliteis"Elv"anabbreviationforElevation.Ifyoucouldstandandpointastraightarmdirectlytowardthesatellite,theelevationwouldbetheangle,indegrees,thatyourarmmadewiththeEarth,assumingthesurfaceislevelwhereyouarestanding.Zerodegreeswouldrepresentasatelliteatthehorizonninetydegreeswouldrepresentasatellitedirectlyoverhead.

{_}Whoops.Timetowritedownanotherlatlonaltposition.

{_}Returntothe"SatPosn&SNR"screen.Thecolumnafter"Elv"isidentifiedas"Az"whichstandsfor



Azimuth.TheAzimuthspecifiestheanglebetweenduenorthandthesatellite:Pointyourarmtowardthenorth,thenrotateyourbodyclockwiseuntilyourarmispointedatthesatellite.Thenumberofdegreesyourbodyrotatedistheazimuth.

{_}Writedownanotherpositionfix.

Thelastcolumnonthe"SatPosn&SNR"screenisthe"signaltonoiseratio"(recallthatitisanindicationofthestrengthofthesignalfromthesatellite).AcceptablevaluesaregreaterthanorequaltofourwhichisthevalueyousetastheSNRmask.Valuesmayrangeupto35orso.

{_}Foreachsatellitebeingtracked,recorditselevation,azimuth,andsignalstrength.

{_}Putyourhandovertheantenna(itisdirectlyunderthelittlesextantembossedintheplasticabovethescreen)andwatchthesignalstrengthdrop.

{_}Determinewhereoneortwosatellitesareinthesky,relativetoyourposition.Trytointerposeyourbodybetweentheunitandasatellitetoseeifyoucanmakethesignalstrengthdropforasinglesatellite.InthemiddlelatitudesintheUnitedStatestherewillgenerallybemoresatellitestoyoursouththannorth

{_}Afterrecordinganotherfix,movetothe"SatHlth&URA"screen.Thisdisplaysthe"health"ofthesatellite,asdeterminedbyinformationbroadcastbythesatelliteitself,andthe"UserRangeAccuracy"(URA)anumericalindicationoftheaccuracyonemightexpectwhenusingthissatellitetocomputeapositionfix.Satellitehealthwillbeindicatedby"OK,"by"U"forunhealthy,orby"n/a"for"notavailable,"indicatingthatnosignalisbeingreceived.

"URA"mayhavevaluesrangingfromoneto1024.Valuesgreaterthan16indicatethattheDoDiscorruptingthesignalfortheparticularsatelliteandthatanysinglepointcalculatedbyusingthatsatellitecouldbeinerrorbyapproximately100meters.ThisisnotsupposedtohappenafterMayDay2000sotheURAvaluesshouldbeontheorderoftwotothree.TheunitsoftheURAnumberaremeters,butsinceagivenpositionisfoundusingacombinationofseveralsatellites,theURAvalueofanyparticularoneisoflimitedusefulnessinestimatingerror.

TheGeo3doesnotdisplayUserRangeAccuracy.

{_}Finishrecordingthe15positionfixes.Istheunitstilltrackingthesamesatellites?Isitusingthesameconstellationofsatellitestocomputefixes?Ifnot,writedownthenewinformation.

Nextpost:BasicConcepts(GPSandGIS)Part5

Previouspost:BasicConcepts(GPSandGIS)Part3

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