The Geocentric Datum of Australia (GDA94) was the official geodetic datum before GDA2020. It was adopted nationally in January 2000. GDA94 replaced the Australian Geodetic Datum 1966 (AGD66) and Australian Geodetic Datum 1984 (AGD84). The standard map projection associated with GDA94 is the Map Grid of Australia 1994 (MGA94), a transverse Mercator projection that conforms to the internationally accepted Universal Transverse Mercator Grid system.

GDA94 is a coordinate reference system whose origin coincides with a determination of the centre of mass of the earth, hence the term ‘geocentric’. GDA94 is a ‘plate-fixed’ or ‘static’ coordinate datum based on the International Terrestrial Reference Frame 1992 (ITRF92), held at the reference epoch of 1 January 1994.

The Geocentric Datum of Australia 2020 Technical Manual contains information about GDA94 and the preferred methods to transform coordinates on GDA94 to GDA2020, AGD84 and AGD66 that most users will require. Further background on how and why GDA94 was established, additional information about its relationship to AGD66 and AGD84 and how to use GDA94 ellipsoidal heights in conjunction with AUSGeoid09 to derive elevations comparable with the national Australian Height Datum (AHD) are found in the GDA94 Technical Manual v2.4

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