GDA94, ITRF & WGS84 What’s the difference? - …

1 gda94 , ITRF & WGS84 what s the difference? Working with Dynamic DatumsGDA94, ITRF & WGS84 what s the difference? Working with Dynamic DatumsRichard StanawayQUICKCLOSEThe assumption gda94 and WGS84coordinatesare identical(for all practical purposes) Movement of the Australian Plate2019181716151413121110 9 8 7 6 5 4 3 2 1 0 Hall, R. 2002. cenozoic geological and plate tectonic evolution of SE Asia andthe SW Pacific: computer-based reconstructions and animations. Journal ofAsian Earth Sciences, 20 (4), 353 434. ITRF/ WGS84 Millions ofYears Close UpThe assumptionWell, they were back in 1994 gda94 and WGS84coordinatesare identical(for all practical purposes) WRONG!!International Terrestrial Reference Frame (ITRF)ITRF is the fundamental reference datum used in geodesyLatest realisation:ITRF2005 Defined by large networks of GPS, SLR, VLBI, DORISC oordinates defined at Reference Epoch + site velocitiesVelocities up to 80 mm/yr due to tectonic motionWorld Geodetic System 1984 ( WGS84 ) WGS84 is the datum used by the GPS SystemLatest realisation.

2 WGS84 (G1150)Defined by coordinates of tracking & control stationsRegularly Aligned with ITRF (at < )1150 = GPS weekGeocentric Datum of Australia ( gda94 )Geocentric datum used in AustraliaGDA94 =ITRF92 (Epoch )Defined by ITRF92 coordinates of the AFNC oordinates fixed at 1st January 1994 valuesNow offset from ITRF and WGS84 by almost 1 metreOffset increasing at up to 70mm/yrNa me _3La t_DeLat_MinLat_SecLong_DLong_MLong_SZone Ea stingNorthingHz_Cl Hz_OOrthometric_HeigDON 95 RM4-29-2 INB ILLA AF-35-23 STUART-19-20 AFN-12-50 AFN-42-48 SPRINGS-23-40 1173-31-52 Coordinates (Geoscience Australia)Offset of WGS84 /ITRF and gda94 in 2007 Arrowsindicatemotion of theAustralianPlate between1994 and 2007 Advantages of gda94 over ITRF/ WGS84 Coordinates do not constantly changeNo requirement for epoch of coordinate acquisitionNo requirement for a velocity modelTendency towards homogeneity of spatial dataPositional uncertainties will decrease over time astechnology & geodetic infrastructure improvesInternal stability of the Australian PlateIntraplate deformation of the Australian continentbetween 1994 and 2007(from baseline analysis of ITRF2005 solution)AustralianDatuminternallyhomoge neousat <10mmover 15 yearperiod(excluding localiseddeformation events)

3 An example of Plate Boundary deformationMw New Ireland EarthquakePapua New Guinea16th November 2000Mw New Ireland EarthquakePapua New Guinea16th November 2000 Deformation at the edge of the Australian Plate(Geological and Nuclear Sciences, New Zealand)Deformationnear Plateboundaries upto 80 mm/yrRequires a dynamicor semi-dynamicdatumfor coordinatestability(as implemented inNew Zealand)Updating gda94 Difference between gda94 andITRF2005 at Epoch could be updated to fit ITRF2005at Epoch 1994 Coordinatedifference( gda94 to update)typically < 30mmin major citiesNGRS benefitsUpdating gda94 Aligning gda94 update with ITRF at currentepoch (2007+) not recommendedOne metre offset would result inconfusion for many usersSpatial datasets would have to be updated(unnecessary cost and effort)Potential for misinterpretation of coordinatesin absence of datum metadataITRF/WGS to gda94 transformation strategiesHigher Accuracy(<15mm)(Australia only)Geoscience Australia14 parameter model( )Medium Accuracy(<30mm)(Any Rigid Australian Plate)Use Global PlateMotion model( APKIM)Lower Accuracy(<50mm)(Location specific within rigid plate only)Use Plate MotionCalculator( UNAVCO)High Accuracy transformation - GA 14 parameter transformationHigh Accuracy(<15mm)Accounts for Reference Frametransformation(from ITRF92 to ITRF2000) 7 parameters (at Epoch )+ 7 parameter rates (per year)Parameters (epoch)

4 = Parameters (2000) + Rate * (epoch - 2000)Computes 7 parameter transformation for any given epochExample of GA 14 parameter transformationITRF2000 coordinates for astation in Hobart at (1st January 2007)X Y coordinatesZone 55E Ht (2007) = + ( )* = (2007) = + ( )* = (2007) = + ( )* = (2007) = + ( )* = ry(2007) = + ( )* = rz(2007) = + ( )* = sc(2007) = + ( )* = parameters for epoch GA 14 parameter Y (ITRF2000 Epoch )EquivalentUTM coordinatesZone 55E Ht to EquivalentMGA coordinatesZone 55E Ht Y ( gda94 )Difference from tabulatedMGA coordinatesE + plate rotation rate( from APKIM)Rotation (epoch) = Rate( /Ma) * * (epoch - 1994)Assumes plate is rigidNot to be used indeforming zones( plateboundaries)Medium Accuracy transformation - Using global plate modelMedium Accuracy(<30 mm)(Actual Plate Kinematic Model, Drewes, H.)

5 Et al, 2002)Example using Global plate model ( APKIM)X Y (ITRF2000 Epoch )EquivalentUTM coordinatesZone 55E Ht to EquivalentMGA coordinatesZone 55E Ht Y ( gda94 )Difference from tabulatedMGA coordinatesE + + Accuracy transformation - Using Plate Motion CalculatorComputesHorizontal velocity forgiven location(dynamic block shift)Easting (MGA) = Easting (ITRF @ epoch) + Velocity* (1994-epoch)Northing (MGA) = Northing (ITRF @ epoch) + Velocity* (1994-epoch)Assumes plateis rigidNot to be used indeforming zones( plate boundaries)( )Example using Plate Motion calculator-42 48 17 147 26 19 EquivalentUTM coordinatesZone 55E (ITRF2000 epoch 2007.)Velocity fromPlate MotionCalculator(ITRF2000 Australian Plate)EquivalentMGA coordinatesZone 55E = + = + (Dynamic block shift)Difference from tabulated MGAcoordinatesE + +( *(1994-2007))+( *(1994-2007))Block shift in HobartCoordinatedifference typically < 30mmin major citiesError arising from using standard Hobart velocityin outer suburbs (between 1994 and 2007)Error escalatesbeyond 20 km(requires newvelocity calculation)ITRF & WGS84 coordinates for locations arechanging constantly in Australia (up to 80mm/yr!

6 Take Home PointsGDA94 & MGA coordinates are fixed to theinternally stable Australian continent ( change<1mm/yr)ITRF & WGS84 coordinates must be transformedusing some dynamic model in order to obtainequiv. gda94 & MGA coordinatesITRF & WGS84 coordinate accuracy ismeaningless unless epoch metadata are provi