Overview of Differences
Geocentric Datum of Australia (GDA94) is Australia’s new coordinate system, which should be used in preference to any others. Based on the International Terrestrial Reference Frame 1992 (ITRF92) it is held fixed at 1 January 1994, hence the name GDA94. However since 1994, the Australian tectonic plate has been shifting approximately northeast at a rate of about seven centimetres per year. Since the whole of Australia is moving uniformly, this movement is not an issue, especially when obtaining positions relative to other GDA94 positions.
However, if a position is obtained directly from a global positioning system (such as GPS) the difference may be apparent. If the global position has a large uncertainty, such as a standard GPS point position with an accuracy of about 10 metres, it is still not an issue. However, when an accurate global position is obtained e.g. a global high accuracy differential GPS system with an uncertainty of a few decimetres, the difference becomes very relevant.
To maintain consistency in Australia, accurate International Terrestrial Reference Frame (ITRF) positions should be transformed to GDA94. This can be done using parameters based on the known GDA94 and continually updated ITRF positions of the Australian Regional GPS Network (ARGN). These parameters are updated regularly and will provide ITRF-GDA94 transformation at a given date, with an accuracy of a few centimetres.
International Terrestrial Reference Frame and WGS84 - GPS SystemThe GPS system (WGS84) has been refined on several occasions since its inception and is now aligned with the ITRF at the level of a few centimetres. For most people it is impossible to obtain a WGS84 position with an accuracy of better than a few metres, but it is now relatively easy to obtain ITRF positions with an accuracy of a few centimetres. (See AUSPOS - our free global GPS processing service). If accurate WGS84 positions are required, it is best to use ITRF positions.
Geocentric Datum of Australia and Australian Geodetic Datum
AGD66 and AGD84 both use the gazetted Australian Geodetic Datum (AGD). The coordinates differ only because of the readjustment using additional data and improved adjustment techniques. The difference between these two coordinate sets varies from about two metres in south east Australia to about five metres in the north west, but the variation is not uniform. The image shows a representation of this variation. Due to the inconsistent nature of AGD66, there are no national transformation parameters available to automatically transform between AGD66 and AGD84.
It may be possible to compute local parameters, but it is most often done by applying a block shift based on a comparison of sites in the area which have both AGD66 and AGD84 positions.
GDA94 is Earth-centred, but the Australian National Spheroid (ANS) used with AGD is offset from the Earth's centre of mass by about 200 metres. This means that AGD and GDA94 positions differ by about 200 metres. Methods to transform from AGD66 and AGD84 to GDA94 are available in chapter seven of the GDA Technical Manual .