2.2 Australian Datums
The current Australian datum is a geocentric datum called the Geocentric Datum of Australia 2020, or GDA2020 for short. Prior to GDA2020, we had GDA94 and before that AGD66 and AGD84.
Australian Geodetic Datum
Australia’s first official national geodetic datum was called the Australian Geodetic Datum (AGD). AGD was initially introduced in 1966, and was based on the local ellipsoid known as Australian National Spheroid or ANS. AGD had two versions: AGD66 and AGD84.
The fundamental characteristics of the AGD datum were:
- ellipsoid – the ellipsoid used was the Australian National Spheroid
- position – the position of this ellipsoid was determined to provide the best fit to the Australian regional geoid
- orientation – the National Mapping Council decided that the ANS should be parallel with the earth’s mean axis of rotation at the start of 1962, and that the meridian plane of zero longitude should be parallel to the zero meridian plane near Greenwich.
Note: the concept of longitude will be fully explained in the chapter on coordinate systems.
The passing of time revealed inaccuracies in the AGD66 coordinates, and this led to a new adjustment of the geodetic network in 1982. The basic definition of the datum was not changed for this purpose – the same ellipsoid was used (ANS) and the origin was held fixed. However, the addition of more observations and the use of improved error modelling and adjustment procedures gave rise to noticeably different coordinates. The resultant coordinate set was termed the AGD84. The comparison of the two datums is shown in Table 2.2(a).
Table 2.2(a): AGD66 compared to AGD84
| Designation | Australian Geodetic Datum (AGD) | |
| Datum | AGD66 | AGD84 |
| Geographic coordinate set | AGD66 | AGD84 |
| Grid coordinates | AMG66 | AMG84 |
| Reference Frame | N/A | |
| Epoch | 1966.0 | 1984.0 |
| Reference Ellipsoid | Australian National Spheroid | |
| Semi major axis (a) value | 6378160.0 | |
| Inverse Flattening | 298.25 |
GDA94
A new and improved datum was adopted by Australia on 1st January 2000. This was known as the Geocentric Datum of Australia 1994 (GDA94).
GDA is a geodetic geocentric datum, based on a model of the whole earth, which has its centre coincident with the earth’s centre of mass. As the words imply, ‘geocentric datum’ is a datum that has its origin at the centre of mass of the earth.
The key advantage of the GDA over Australia’s earlier datum (AGD) is that GDA is totally compatible with satellite-based navigation systems such as GPS and with major international geographic systems.
GDA is the datum used in Australia, while GDA94 was the geodetic coordinate set (latitudes and longitudes) computed in terms of the datum at 1 January 1994. The reference frame for GDA94 was the International Terrestrial Reference Frame (ITRF1992), at epoch 1994.0.
The fundamental characteristics of the GDA datum, adopted by a meeting of the Intergovernmental Committee on Surveying and Mapping (ICSM) held in