Geocentric Datum Of Australia 1994 (GDA94)
The Geocentric Datum of Australia (GDA94) is the official geodetic datum adopted nationally across Australia on 1 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 that best fits the shape of the earth as a whole. It has an origin that coincides with the centre of mass of the earth, hence the term ‘geocentric'’ GDA94 is a static coordinate datum based on the International Terrestrial Reference Frame 1992 (ITRF92), held at the reference epoch of 1 January 1994.
It is considered to be the most effective datum as it provides:
- compatibility with satellite navigation systems, such as the Global Positioning System (GPS)
- compatibility with national mapping programmes carried out on a geocentric datum,
- single standard for the collection, storage and dissemination of spatial information at global, national and local levels.
The AGD provided a reference system that best fit the shape of the earth in the Australian Region but its origin did not coincide with the centre of mass of the earth. National datums were commonly non-geocentric before satellite based navigation systems were established in the early 1970’s
The distance between the origin points of GDA and AGD is approximately 200 metres. When the coordinates of a point on the Earth’s surface are converted from AGD to GDA this translates to a coordinate difference of approximately the same amount. The difference varies slightly depending on where you are in Australia.
Information Video and Reference Material
In 1999 ICSM produced a CD which contained a package of information videos and reference material. This was revised in 2002. A selection of the material form this CD can be accessed here:
- View the video (86.6Mb)
- Geocentric Datum of Australia Technical Manual (.pdf file, 0.9 MB)
- Where in the World are We? - South Australian Department for Environment & Heritage (.pdf file, 0.9 MB)
- Brochure: Know Where You Stand with GDA, ICSM, October 1997 (.pdf file, 2.9 MB)
- Brochure: Get In Step With the Geocentric Datum - Discussing the Business Issues, June 1999 (.pdf file, 1.9 MB)
- Final report on the Development of the National Transformation Grids (.pdf file, 2.5 MB)
- GDA Contacts
Implementing GDA - Background Information
Background information about the decision making and implementation of GDA can be accessed at the GDA History page.
Implementing GDA - Adjustment of the Combined State and Territory Geodetic Networks
The GDA coordinates of the Australian Fiducial Network (AFN) of permanently tracking GPS sites, and the Australian National Network (ANN) of existing GPS coordinated survey marks, were determined by the processing of global GPS observations.
The next stage of the GDA implementation required the existing geodetic survey networks to be adjusted to the framework provided by the AFN and ANN. Initially this project involved just a few ICSM jurisdictions and the project was colloquially referred to as ‘the spine’ adjustment. Other jurisdictions subsequently joined the project so that this adjustment now includes all regions except Western Australia. More information can be found at the adjustment of the combined State and Territory geodetic networks (spine) page.