Geocentric Datum Shift (GDS)

• Introduction to the Geocentric Datum of Australia, GDA 94
• What is the Geocentric Datum?
• What are the benefits?
• How will GDA affect legal boundaries?
• Why change?
• How will it affect me?
• Where in the world are we?

Documents for download from this site are in PDF format and you will need Adobe Acrobat Reader to view them. The reader is free and can be downloaded from the Adobe website.

Introduction to the Geocentric Datum of Australia, GDA 94

On January 1^st 2000, GDA 94 was adopted by the states and territories of Australia as the official datum for all spatially related data.

This change affects everyone who uses mapping and spatial data. While the features on the ground will not alter, their coordinate values will change by approximately 200 metres in a north easterly direction.

What is the Geocentric Datum?

Since the dawn of time, man has found it necessary to measure and map his domain. The Earth is poorly approximated by a sphere as it is significantly wider at the equator than between the poles. Over the years various methods have been adopted to facilitate mapping the earth’s features and this has presented many challenges. A geocentric datum is a reference system that has as its origin the Earth’s centre of mass, and is the best fitting reference surface for the whole earth.

What are the benefits?

It is directly compatible with satellite-based navigation systems. Positioning from these satellites is a critical element in modern surveying, mapping, geographic information systems, navigation and aviation. The adoption of a geocentric datum allows for a single standard for the collection, storage and use of geographic data, ensuring compatibility across various geographic information systems at the local, national and global level.

How will GDA affect legal boundaries?

Legal boundaries are generally defined by physical features or marks on the ground. These boundaries will not change, though the coordinates, if utilised in defining location, may alter.

Why change?

The current Australian Geocentric datum (AGD 84) is based on a mathematical surface of the Earth designed to fit the local Australian region. Continuing to work with AGD coordinates in an international environment is becoming increasingly inefficient and difficult.

How will it affect me?

Generally, it had no direct effect, however, the conversion to GDA is most noticeable on mapping products and you should be aware of this when reading coordinate values from a map or using a GPS.

Please email Geoff Sandford for detailed information and additional explanatory material including a free video.

Where in the world are we?

A document providing a brief introduction to the development of spatial reference systems, addressing some of the technical issues is available for download (163Kb PDF).