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BasicConcepts(GPSandGIS)Part2

HOWWEKNOWWHERESOMETHINGIS

First,adisclaimer:Thistextdoesnotpretendtocoverissuessuchasgeodeticdatums,projections,coordinatesystems,andothertermsfromthefieldsofgeodesyandsurveying.Infact,Iamgoingtomakeitapointnoteventodefinemostoftheseterms,becausesimplyknowingthedefinitionswillnotserveyouunlessyoudoagoodbitofstudy.Thesefields,concepts,andprinciplesmayormaynotbeimportantinthecollectionofGPSinformationforyourGISusedependingonthesortofprojectyouundertake.Whatisimportant,vitalinfact,isthatwhenGPSdataaretobecombinedwithexistingGISormapinformation,thedatumdesignation,theprojectiondesignation,thecoordinatesystemdesignation,andthemeasurementunitsthatareusedmustbeidentical.

BeforeweundertaketouseaGPSreceivertodetermineaposition,itisimportanttounderstandwhatismeantbythatterm.Itseemslikeastraightforwardidea,butithasconfusedalotofpeople,particularlywhenagivenpositionisdescribedasasetofnumbers.7

Takeatentstake,oraknittingneedle,oratableknifeandstickitverticallyintoapatchofground.Nowreflectuponthefactthattheobjectisnotmovingwithrespecttotheplanet,8asitwouldbeifyoustuckitintothesoilinaflowerpotonthedeckofaship.Thelocationoftheobjectmaybeidentifiedthroughtimebythreeunchangingnumbersoflatitude,longitude,andaltitude.Inotherwords,itiswhereitis.Butinthelastcentury,teamsofmathematiciansandscientists(skilledingeodesy)havedevelopedothersetsofnumberstodescribeexactlythesamespotwhereyourobjectnowresides.Thespotspositiondoesntchange,butthedescriptionofitdoes.

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Ignoringthematterofaltitudeforthemoment,supposeyouputyourobjectintothegroundatlatitude38.0000000(North)andlongitude84.5000000(West),accordingtothelatestbeliefaboutwherethecenteroftheearthisandwhereitspolesare.Thismostrecent,widelyacceptedviewisthedatumdescribedastheWorldGeodeticSystemof1984(WGS84),basedontheGRS80ellipsoid.9InthecoterminousstatesoftheUnitedStates,thisdatumissaidtobevirtuallyidentical(withinmillimeters)withtheNorthAmericanDatumof1983(NAD83).AsIindicated,ifyouwanttoknowmorethereareplentyofsources.

Priortothislatestdeterminationofthelatitudelongitudegraticule,manypeopleandorganizationsintheUnitedStatesusedtheNorthAmericanDatumof1927(NAD27)asthebestestimateofwherethelatitudelongitudegraticulewaspositioned,basedonparametersdeterminedbyClarkein1866.Accordingtothisdatum,yourobjectwouldbeatlatitude37.99992208andlongitude84.50006169.Thislookslikeaninsignificantdifferencebuttranslatesintoabout10metersontheearthssurface.Toconsideranotherwayofthinkingaboutwhatthismeans:Ifyouputasecondobjectinthegroundat38latitudeand84.5accordingtoNAD27,itwouldbe10metersawayfromthefirstone.Doesthatsoundlikealot?Peoplehavebeenshotindisputesovermuchsmallerdistances.So,whenaGPSreceivergivesyoualatitudeandlongitudeyoumustknowthedatumthatisthebasisforthenumbers.Therearehundredsofdatumsalmosteverycountryhasitsown.NAVSTARGPSisbasedfundamentallyonWGS84butlotsofGPSunits,includingtheonesyouwillbeusing,cancalculateanddisplaycoordinatesinmanyotherdatums.

Therearemanyreasonswhypeopledonotfinditconvenienttouselatitudeandlongitudetodescribeagivenpoint,orsetofpoints,ontheearthssurface.Oneisthatdoingcalculationsusinglatitudeandlongitudesay,determiningthedistancebetweentwopointsisaprettycomplexmatterinvolvingsuchthingsasproductsofsinesandcosines.Forasimilardistancecalculation,ifthepointsareonaCartesianplane,theworstarithmetichurdleisasquareroot.

Anotherreasonnottouselatitudeandlongitudemeasuresformanyapplicationsisthatitdoesntworkwellforseveralaspectsofmapmaking.SupposeyouplotmanypointsontheEarthssurfacesaythecoastlineofasmallislandsomedistancefromtheequatoronapieceofordinarygraphpaper,usingthelongitudenumbersfor"x"coordinatesandlatitudenumbersfor"y"coordinates.Theshapeoftheislandmightlookprettystrangecomparedwithhowitwouldappearifyougotupinanairplaneandlookeddownonit.Andifyoumeasureddistancesoranglesorareasontheplotyouwouldnotgetusefulnumbers.Thisisduetoacharacteristicofthesphericalcoordinatesystem:Thelengthofanarcofadegreeoflongitudedoesnotequalthelengthofanarcofadegreeoflatitude.Itcomesclosetoequalneartheequatorbutitisnowherenearcloseasyougofurthernorthorsouthfromtheequator.Attheequatoradegreeoflongitudeisabout69.17miles.Verynearthenorthpoleadegreeoflongitudemightbe69.17inches.(Adegreeoflatitude,ontheotherhand,variesonlybetweenabout68.71milesneartheequatorand69.40milesnearthepoles.)

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Agoodsolutiontotheseproblems(calculationandplotting)forrelativelysmallareasistheconceptofa"projection"withwhichyouarenodoubtfamiliar.Thetermcomesfromimaginingaprocessinwhichyouplacealightsourcewithinatransparentglobethathasfeaturesinscribedonit,andletthelightfallonaflatpieceofpaper(oronethatiscurvedinonlyonedirectionandmaybeunrolledtobecomeflat).10Theshadowsofthefeatures(say,linesorareas)willappearonthepaper.YoucanthenapplyaCartesiancoordinatesystemtothepaper,whichgivesyoutheadvantagesofeasycalculationandmorerealisticplotting.However,distortionsareinherentinanyprojectionprocessmostofthepointsonthemapwillnotcorrespondexactlytotheircounterpartsontheground.Thedegreeofdistortionisgreateronmapsthatdisplaymorearea.

Insummary,latitudeandlongitudenumbersofagivendatumprovideanexactmethodofreferencinganygivensinglepoint,buttheyaredifficulttocalculatewith,andmultiplepointssufferfromdistortionproblemswhenplotted.Projectedcoordinatenumbersareeasytocalculatewithbut,ingeneral,misplacepointswithrespecttootherpointsthusproducingerrorsinthedistances,sizes,shapes,anddirections.

Letslookatsomecommonrepresentationsofthepositionofourobject.

AcoordinatesystemcalledUniversalTransverseMercator(UTM)wasdevelopedbasedonaseriesof60transverseMercatorprojections.Theseprojectionsarefurthersubdividedintoareas,called"zones,"covering6oflongitudeand,usually,8oflatitude.Acoordinatesystemisimposedontheresultingprojectionsuchthatthenumbersarealwayspositive,andincreasing"totherightandup."

Therepresentationofourobject(at38Nand84.5)intheUTMcoordinatesystem,whenthatsystemisbasedonWGS84,isa"northing"of4,208,764.4636metersandan"easting"of719,510.3358meters.Thenorthingisthedistancealongthesurfaceofa"bumpless"earth,inmeters,fromtheequator.Theeastingissomewhatmorecomplicatedtoexplainsinceitdependsonthezoneandacoordinatesystemthatallowsthenumbertoalwaysbepositive.

However,thereisalsoaversionoftheUTMcoordinatesystembasedonNAD27.Inthiscase,ourobjectwouldhavedifferentcoordinates:northing4,208,550.0688andeasting719,510.6393,whichmakesforadifferenceofabout214meters.IfyoucomparethesecoordinateswiththepreviousUTMcoordinatesyouseethatvirtuallyallofthedifferenceisinthenorthsouthdirection.(Whilethisistrueforthisparticularposition,itisnottrueingeneral.)ObviouslyifyoutriedtocombineWGS84UTMdatawithNAD27UTMdatathelocationstheydepictwouldnotmatch.

AllstatesintheUnitedStateshaveStatePlaneCoordinateSystems(SPCSs)developedbytheU.S.CoastandGeodeticSurvey,originallyinthe1930s.Thesesystemssometimestwoormorearerequiredforagivenstatearebasedondifferentprojections(mostly,transverseMercatororanothercalledLambertConformalConic)dependingonwhetherthestateismostlynorthsouth(likeCalifornia)ormostlyeastwest(Tennessee).Theunitsareeitherfeetormeters.Zoneboundariesfrequentlyfollowcountyboundaries.Asyoucaninfer,eachstateoperatesprettyindependentlyandthecoordinatesystem(s)usedinonestatearenotapplicableinitsneighbors.InKentucky38.0000000(North)andlongitude84.5000000(West)wouldtranslateintoanorthingof1,568,376.1900feetandaneastingof182,178.3166feetwhenbasedonWGS84.However,whenthebasisisNAD27,thecoordinatesare1,927,939.8692and182,145.9821,whichmakesadifferenceofsome68miles!

WhythelargedifferencesincoordinatesystemsbasedonNAD27versusthosebasedonWGS84?BecausethoseresponsiblefortheaccuracyofothercoordinatesystemstookgoodadvantageofthedevelopmentofWGS84aworldwide,earthcentered,latitudelongitudesystemtomakecorrectionsorimprovementstothoseothersystems.

Stateplanecoordinatesystemshaveascaleerrormaximumofabout1unitin10,000.SupposeyoucalculatedtheCartesiandistance(usingthePythagoreantheorem)betweentwopointsrepresentedinastateplanecoordinatesystemtobeexactly10,000meters.Then,withaperfecttapemeasurepulledtightlyacrossanidealizedplanet,youwouldbeassuredthatthemeasuredresultwoulddifferbynomorethanonemeterfromthecalculatedone.ThepossiblescaleerrorwiththeUTMcoordinatesystemmaybelarger:1in2,500.

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STEPBYSTEP

Disclaimer

TheStepbyStepsectionsofthistopicdependonhardware,firmware,andsoftwarecreatedbyvariousmanufacturers.Asyouknow,suchentitiesmaychangeovertime,slightlyorradically,asbugsarefound,improvementsaremade,orforotherreasons.AlltheStepbyStepprocedureswerecheckedoutintheearlymonthsof2001andfoundtowork.Thedisclaimeris:thereisnoguaranteethattheywillworkforyouexactlyasintended.Youmayhavetobeflexibleandlookforworkaroundsorfinddifferentmenuitems.TheHelpfilesmayhelptheymaynot.But,anyway,youwouldntbetryingtousecomputersifyouwerentflexible,wouldyou?

Preparation

FortheStepbySteppartofthistopicyoushouldobtainatopomap(UnitedStatesGeologicalSurvey[USGS]topographicquadrangle),ifyouareintheUnitedStates,oftheareainwhichyouareplanningtocollectdata.ThismapwillalsoindicatetheUniversalTransverseMercator(UTM)zone(e.g.,16T),whichyoushouldwritedownforuseinPROJECT1C.

Also,seeifyoucanfindanearbysurveymarkerwithknowncoordinates.(AwebsitelistingallU.S.CoastandGeodeticSurveymarkersmaybefoundatwww.ngs.noaa.gov.11Clickonthehyperlinkthatinvitesyoutogetthedatasheetsrelatedto"Findapoint.")

PROJECT1A

GettingAcquaintedwithaGPSReceiver

YoubeginyourfirstGlobalPositioningSystemprojectbybecomingacquaintedwithatypicalGPSreceiver,whilestillinsideabuilding.YourinvestigationbeginswithaGeoExplorer12receiver,andanotebook.

TheGeoExplorerandtheenhancedversion,theGeoExplorerII,operateinmuchthesamefashion,soIwillsimplyusethenameGeoExplorertorefertobothunits.

Theprimaryexternaldifferenceisthatthe"II"hasareceptaclethatallowsuseofanexternalantenna.Theoriginalreceiverwasbuiltwithonlyaninternalantenna,makingitlessconvenientinsomesituationsasinsideanautomobile,forinstance.

ThenotebookwithpaperandasturdywritingsurfaceistherepartlytogetyouusedtotheideathataGPSreceiverandanotetakingabilitymustgohandinhand.Afterthisfirstproject,mostofthedatayoutakewillberecordedincomputerfiles,butsomewillnotandmustbewrittendown.Thenotebookwillalsoprovideanindextothecomputerfilesyourecord.

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YoumaybeusingaGeoExplorerS(calledGeo3inthistexttodifferentiateitfromGeoExplorersIandII),whichisamuchdifferentdeviceintermsofhowitisoperated,butwhichproducesthesameresults.TheGeo3comeswithaCDROMthatcontainsatutorialthatdescribesindetailhowtouseitsoIwillnotdosointhistext.InsteadIwillassumeyouhavesomeknowledgeoftheGeo3fromgoingthroughthetutorialontheCDROM.However,IwillgivesummaryinstructionsforuseoftheGeo3inboxessuchasthissoyoucandothesameprojectsaswiththestandardGeoExplorer.

IfyouareusingtheGeo3youshouldfirstreadtheinstructionshereinfortheGeoExplorerandthenadaptthemfortheGeo3,withguidancefromtheseboxes.

AGeoExplorerreceiver,withitsbuiltinantenna,isahandhelddeviceabout3.75Dwideby7Dlongandlessthan2Dthick,andweighslessthanapound.ItisusuallypoweredinthefieldbyastandardVideoCambatterythatweighsabout1.5pounds.Alternativepowersourcesare"AA"batteriesandtheauxiliarypower(cigarettelighter)receptaclesinacar,boat,orairplane.

Whiletheunitisverycomplexinternally,containingnotonlysignalreceptionelectronics,butamicrocomputerwithaquartermegabytememoryaswell,itsusercontrolsarequitebasic:eightbuttons.SeeFigure13.

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Figure13.TheGeoExplorerIIGPSreceiverfromTrimbleNavigation.

TheGeo3containsitsownbatteries.Whileanexternalpowerkitisavailable,theinternalbatterieslastformanyhoursandareprobablysufficientformostfieldwork.Sinceitcontainsitsownbatteriesitisalsoheavier.SeeFigure14.

TheGeo3containsamegabyteofmemory.Itisasomewhatlargerunitwithamuchbiggerscreen.

Thebuttonthatis"furthestsouth"istheonoffcontrol.The"up"and"down"buttonsscrollthroughthelinesoftextandnumbersofwhateverscreeniscurrentlydisplayed.The"left"and"right"buttonsmoveacursorfromcharactertocharacterwithinastringofcharactersandperformotherselectionfunctions.Thecenterbuttontheonewiththediamondisthe"Command"keywhichsetsoptionschosenbytheothers.Wewilldesignatethiskeyas"CMD"fromnowon.

Visualoutputtotheuserisprovidedbyafourline,16characterperlineLCDdisplay.Inmanyinstancesthescreenservesasawindowtoalistofmorethanfourlines,buttheusercanseeallofthembyrepeatedlypressingthe"down"key(orthe"up"key).

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Figure14.TheGeo3GPSreceiverfromTrimbleNavigation.

PowerOnandOff

{__}13

{_}Findthecordwiththesmallcylindricaljackononeendandthelargecylinderontheother.Connectittothereceiverandthebattery(orbatterycharger).Asmallgreenlightatthebaseofthereceivershouldglow.Thescreenofthereceivermaybeblank,ormaydisplaycharacters.Ifyouseecharacters,holddowntheonoffbuttonuntilthedisplaybecomesblank.

{_}Presstheonoffbutton.Somepreliminaryscreenswillappearthatdescribethereceivertype,firmwareversion,andotherfacts.Notetheseitemsinyournotebook.(Ifyoudontgetallofthemthistime,checkthenexttimeyouturntheuniton.)

MainMenu

1.DataCapture

2.Position

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3.GPSStatus

4.Navigation

5.Date&Time

6.Configuration

7.DataTransfer

ToturnontheGeo3simplypressthe"poweron"buttonlowerrighthandcorner.

{_}Whatyouseenowdependsonthestatethereceiverwasleftinbytheprevioususer.YourgoalatthispointistoreturntotheMainMenu.Press"Esc"untilitappears.Here,forexampleandforfuturereference,istheMainMenu.

{_}Pressandholdtheonoffbutton.Thereceiverwillnotimmediatelygooff.Rather,ascreenwillappearthattellsyouthat"OFF"isimpendingin5,4,3,etc.seconds.This"delayedoff"servesapurpose.Incaseyouarerecordingdataandaccidentallyattempttoswitchthereceiveroff,yougetachancetorecoverfromthiserrorbysimplyreleasingthebutton.(If,whilethescreeniscountingdown,youpress"CMD,"thereceivergoesoffimmediately.Practicethatnow.)

{_}Turnthereceiveronagain.Nowturnitoffanddisconnectthepowercordatthe"cigarettelighter"junction.Youshoulddothiseverytimeyouturnthereceiveroffunlessyouplantouseitagainwithinaquarterofanhour.Thereceivertakespowerfromthebatteryeventhoughitisoff.Youknewthatanyway,didntyou?Thatlittlegreenlightatthebaseofthereceiverwontglowallonitsown.

{_}Reconnectthepowercordandturnthereceiveronagain.Darkentheroomormovetoarelativelydarkplace.Holdtheonoffswitchdownandpressthekeywiththebentarrowonit.Notethatthiscausesascreen"backlight"tocomeon.Repeattheprocesstoturnthebacklightoff.Beawarethat,ifyouareusingbatterypower,thebacklightreducestheamountoftimeyoucanusetheunitwithoutrechargingthebatterypack.

{_}Withthereceiveron,pressandholdtheonoffkeyandthe"up"keytogether.Notethechangeincontrastonthescreen.

Releasebothkeys.Nowtrythesamewiththe"down"key.Youcanmakethescreenmorereadablebyadjustingthecontrast,dependingonthelightingconditionsaroundyou.(Thiscontrastadjustmentprocessmayalsoturnonthebacklight,sobecarefulnottoleavethebacklightonanddrainthebattery.)Turnthereceiveroff.

{_}Nowcyclethroughthisonoffprocedureacoupleoftimes,employingthebacklightalso,untilyougetcomfortablewithit(andhavenoteddowntheinformationontheinitialscreens).

OntheGeo3thebacklightissetbyfirstpressing(orpressingandholdingdown)theblueFnbutton(whichtellsthereceivertoshiftintoadifferentfunctionmode),andpressingthebuttonoverthelightbulbicon(i.e.,theDATAbutton).ScreencontrastisalsosetwiththeFnkeyandappropriatebuttonsovericons.TheshorthandusedinthistextforpressingFnfollowedbyabuttonisFn&.Forexample,toincreasethescreencontrastpressFn&NAV.TobringuptheoverallmenuoftheGeo3,pressFn&OPTION.

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TheSourcesofPowerandOtherGadgets

{_}Whileyouareexploringthereceiver,getacquaintedwiththeotherparaphernaliathatcomeswithit.

Theotheraccessoriesyoushouldfindare:

Thebatterycharger,withtworedLEDindicators.Youmayleavethebatteryconnectedtothechargerindefinitely.Thebatteryisfullychargedwhenthe"FASTCHARGE"LEDgoesout.

Acablethatcanconnectthereceivertoacomputer.Atthereceiverendisacirculareightpin,maleplug.Attheother(computer)endisastandardninepinRS232serialportconnector.

SomePCcomputersoftwarenamedPathfinderOfficeandaprinterporthardwarekey,tobediscussedlater.

ManualsrelatedtotheGeoExplorer:OperationManual,SoftwareUserGuide,andaGeneralReference.

A"Y"cablewithtwofemalesocketsandamaleplug.YoumayusethiscabletoconnecttheGeoExplorerdirectlytothebatterycharger,thussavingthebatterywhenyouareindoorswiththeunitturnedon.

Youhavealreadymettherechargeablebatterypack.Fullycharged,itshouldsupplymorethaneighthoursofreceptionifthebacklightisnotused.Ittakesaboutthesameamountoftimetochargeasitdoestorundown.

AsecondsourceofelectricityfortheGeoExplorerreceiverisadetachablebatterypackthatholdsfour"AA"sizebatteries.Usingdisposablealkalinebatteries,itpowersthereceiverforabouttwohoursifrechargeablebatteries,orstandardflashlightbatteriesareused,theoperatingtimeisless.

Wheneitherofthebatterypacksisalmostexhausted,thescreenfadesandthereceivershutsitselfdown.Almostnonoticeisgivenbeforethereceivershutsoff,sotreattheamountofchargeinthebatterypackconservatively,asyoumighttheamountofgasinyourautomobiletankonalongtrip,withfewfuelingstationsalongtheway.Forinformationaboutthebatterypack(warnings,howtochargeit,itsabilitytoholdachargewhileontheshelf,orreplacingthe"AA"cells),seethereceiveroperatingmanual.

Athirdoptionforoperatingthereceiveristoplugthepowercordintoacar,boat,oraircraftDCpoweroutlet.Theconnectorisdesignedtofitastandardcigarettelighter.Theacceptablerangeofvoltageinputisbroad:from9to32volts.Whilethisisagoodwaytopowerthereceiverwhensuchpowersourcesareavailable,youmustbecarefulnottostartorstoptheengineofthevehiclewhiletheunitispluggedin,whetherornotthereceiverisoff.Startinganengine,inparticular,caninducevoltagespikesthatcandamagethereceiver,evenifitisturnedoff.

Afourthoption,usableonlywhenyouareneara110voltACpowersource,istoconnectthereceivertothebatteryrechargerbywayofthe"Y"powercable.Youcanthenreviewanduploadfilestoacomputerwithoutusingupbatterypower.YoucanalsosavebatterypowerwhileconfiguringtheGPSreceiver.Itmightbeagoodideatodothisnowsinceyouhaveabout30minutesofworkaheadofyouasyouconfigurethereceiverbeforeyoutakeitintothefield.

TheGeo3comeswithacradlethatmaybeconnectedtobothastandardACpowersourceandtoacomputer.Whenthereceiverisplacedinthecradleitautomaticallybeginsrecharginganddatamaybetransferredbetweenitandthecomputer.

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