Geocentric Datum Of Australia (GDA)

The Geocentric Datum of Australia (GDA) 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'.

Following a resolution of the Intergovernmental Committee on Surveying and Mapping (ICSM) in 1988, it is being progressively implemented throughout Australia as the preferred datum for all spatial information.

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 already carried out on a geocentric datum,
  
  
• single standard for the collection, storage and dissemination of spatial information at global, national and local levels.

GDA replaces the Australian Geodetic Datum (AGD) which has been in place since 1966.  The AGD provided a reference system that best fitted 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.

Implementing GDA - Background Infromation

Background infromation about the decision making and implementaion of GDA can be accessed here: GDA History.

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 about the adjustment of the combined State and Territory geodetic networks (spine) can be found here.

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