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Clarification on elevation data referenced to AHD when converting between GDA94 and GDA2020

The following clarification is provided following several enquiries from industry

Heights such as Ground Level Elevations and Kelly Bushing Elevations etc. for petroleum-related data are usually stated with reference to the Australian Height Datum (AHD).  AHD is the official national height reference datum for Australia and was adopted in 1971.  It is based on mean sea level (MSL) observations at various mainland tide gauges around Australia.  AHD has not been modified during the transition from GDA94 to GDA2020, hence heights such as the GL elevation or KB elevation for a well (referenced to AHD) will be the same in both GDA94 and GDA2020.

Confusion sometimes arises because GDA94 and GDA2020 are based on different realisations of the International Terrestrial Reference Frame and hence, for any given point, the height measured above the ellipsoid will vary by approximately 9cm between GDA94 and GDA2020. However, when an ellipsoidal height is converted to an AHD height (which has not changed during the transition from GDA94 to GDA2020), different geoid models must be used and these back out the effect of the change in the ellipsoid. The AUSGeoid09 model must be used when dealing with GDA94 co-ordinates and GDA94 ellipsoidal heights while the AUSGeoid2020 model must be used when using GDA2020 co-ordinates and GDA2020 ellipsoidal heights.

More information can be found on Geoscience Australia’s Australian Height Datum and Geoid Models webpage.

For more information about DEM's Transition to GDA2020 including guidance and background see the DEM Transition to GDA2020 page.