WGS84与ITRF之间的对应关系

WhatDatumAmIOnJamesR.ClynchMarch2006I.TheBasicQuestionsBecausecoordinatesindifferentdatumscandifferby100'sofmeters(orevenakilometerinthefareast),itisimportanttoknowwhatdatumyouareon.Thereareusuallytwoquestions:Whatdatumismymapordatabaseon?Whatdatumismynavigationsystemorsurveyon?Clearlyiftheansweristhesame,themap/databaseandthepositionsensorcanworktogether.Shipshavegoneagroundwhenthiswasnotso.TodaywearemovingtotheuseoftheGlobalPositioningSystem(GPS)andelectronicchartsandmaps.Howevertherearestillplentyofpapermapsaround.AndevenelectronicchartsdirectlyconnectedorintegratedwithGPSdonotguaranteedthatwearenotmakingadatumerror-combiningmaterialfromtwoormoredatums.Thisnotewilltryandshedsomelightonwhatdatum(ordatums)youareusing.II.MapDatums-PaperandElectronicThedatabaseofamapisusuallylistedinthelegendofthemap.Infactthereareisusuallybothahorizontalandverticaldatumlisted.Todayitisnotuncommontoseetwohorizontaldatumslisted,onefortheoriginalmapandoneforsomeoverprinting.ThisishowUSGShasupdatedalotoftopographicalmapsfromNAD27toNAD83.Butyouhavetoreadthelegendcarefullytonoticethis.InfacttherearequiteafewmilitarymapsissuedbyNIMAthatusethesametechnique.ThemilitarymapsoftheBalkansdistributedinthemid1990'swereontheEuropeanDatumof1950withannotationsinthelegendsonhowtoshiftthepositionstoWGS84.Forcomputerdatafilestheissueismuchmoredifficult.Thedatafromthelegendsofthemapsisusuallypreserved,butoftennotdisplayed.Ifthemapissimplyscannedasanimagethelegendisthere,butthedataisnotusuallyregisteredorsetupforcomputerreadingofaccuratecoordinates.IfthemaphasbeenenteredintoaGeographicalInformationSystem(GIS)itmaywellberegistered,butinthiscasethelegenddataispresentonlyinanauxiliaryfile.Thistypeoflegenddataiscalledmetadata.III.NavigationandSurveyEquipmentDatumsWhatdatumisGPSon?TheanswerdependsonhowtheGPSreceivergeneratedthesolution.StandaloneGPSandDifferentialGPS(DGPS)havedifferentanswers.Andtherearebothageneralanswer,andamorepreciseanswer.A.StandaloneGPSUsersSimplyput,standaloneGPSusersareonthedatumoftheephemeristhattellsthesolutionprogramthelocationsofthesatellites.Thisisthedatumofthestationsusedtocollectthedatafortheephemeriscomputation.ForthestandaloneuserthesimpleanswerisWGS84.GPSoperatesbymeasuringrangesfromthesatellitestotheuser.Inordertoconverttheserangesintopositions,thelocationsofthesatellitesatthetimesthemeasurementsweremadeisneeded.Havingarangeandnotknowingwhereitisfromisnotuseful.Thetimehistoryofthesatellitepositionsiscalledtheephemerisofthesatellites.Soastandaloneuserisonthedatumoftheephemerisheuses.Theusualcaseisfortheusertousetheephemeristhatispresentonthesignalbroadcastbythesatellites.ThisiscalledtheBroadcastEphemeris(BCE).ThisinformationisonWGS84.Thequotesdenotethatthisanswerisnotpreciselycorrect,oriscomplicatedbyhistory.TherehavebeenPreciseEphemeris(PE's)availableforover20years.Thesearepostfitephemerisbasedonalargesetofgroundstations.Theseareavailablefromseveralcivilianandmilitarysources.ThereisevenacivilianconsortiumthatproducesablendedsetofseveralPE's.InthepastthesewereusedforpostfitworkbecauseitwasaweekormoreafterthefactthatthePE'sbecameavailable.After2000,theyhavebecomeavailableatveryshortdelays.ThereevenisarapidpredictionservicethatgeneratesprojectedPE'sthataremuchbetterthanBCEouttoadayorso.PE'sareusuallyonthelatestInternationalTerrestrialReferenceSystem-ITRF2000in2002.Ifyouusetheseephemeris,youareonITRF2000.ThemorecomplexanswerfortheBCE'salsoputstheseonaITRF.AnephemerisiscomputedfromGPSobservationsmadeatknown,fixedlocations.Thedatumofanephemerisisdeterminedbythecoordinatesusedfortheantennasoftheseobservations.TheselocationsfortheOperationalControlSystemstationshavebeenadjustedseveraltimes.ThishaseffectivelychangedthedatumofWGS84asrealizedbytheGSPBCE's.Inordertoavoidconfusion,thenameofthedatumwasnotchangedwhentheadjustmentsweremade.Asuffixwasadded,butnotwidelyusedorknowoutsidethegeodesycommunity.TheBCEdatumswere/are:BCEDatumNameImplementedMatchesWGS841980OriginalWGS84WGS84(G730)1-2-1994ITRF94WGS84(G873)9-27-1996ITRF96WGS84(G1150)1-2002ITRF2000ThenumberintheGnnnistheGPSweeknumberofthechange.GPStimeiscountedinweeksafterJanuary1980.B.DifferentialGPSUsersSimplyput,forDGPSyouareonthedatumusedbythereferencestationreceiver.WithDGPS,areferencestationataknownlocationmeasurestheerrorsinrangestoindividualsatellitesandsendsthesetoremoteusersoversomecommunicationlink.Inordertocomputetheerrors,thereferencestationneedstoknowitsposition.ThedatumofthecoordinatesusedforthispositionbecomethedatumofallDGPSsolutionsthatusethosecorrections.C.OtherElectronicNavigationSystemsOtherelectronicnavigationsystemsaremuchlikeDGPS.Thecoordinatesusedforthestationsthattransmitthesignalsortranspondersignalsdefinethedatum.Oftenthesearenotaccurateenoughformeterleveldistinctionstobeimportant.HoweverinAsia,thedifferencebetweenWGS84andsomeotherlocaldatumscanbelarge.Theusermusttakecaretoknowwhichdatumhisnavigationsystemandmapareon.D.SurveyingSurveyingismuchlikeDGPS.Surveyingisusuallydonepointtopoint.Inasurveyednetworktheremustbeatleastoneknownpoint.Inlargesurveysth