Geodetic datums are models for different regions of Earth that give greater accuracy to measurements of longitude and latitude, altitude, and elevation. Datums are used in fields such as navigation and surveying to translate locations on maps to their real position on Earth. Although there is a world datum (WGS84), local datums can be more accurate because Earth is not perfectly spherical.

Because a particular datum is developed from different parameters to define Earth’s shape, the same location will have different coordinates, depending on which datum has been used.

Horizontal datums

A horizontal datum describes a location on Earth’s surface in a coordinate system such as latitude and longitude.

In Australia, 4 datums have been defined:

• Australian Geodetic Datum 1966 (AGD66), which uses the parameters defined by the Australian National Spheroid
• Australian Geodetic Datum 1984 (AGD84), which uses the parameters defined by the Australian National Spheroid
• Geocentric Datum of Australia 1994 (GDA94), which uses the parameters defined by Geodetic Reference System 1980, and replaces AGD66 and AGD84; adoption of the system began in 2000

• Geocentric Datum of Australia 2020 (GDA2020), an update of GDA94 to account for Australia's further movement to the northeast and improved satellite positioning technology; adoption from 30 June 2020.

Each datum uses a reference datum (a local reference point) that is used to describe other locations. Use initial capitals for full datum names, and do not include a space between the letters and numbers in the abbreviated form:

Vertical datums

Vertical datums measure elevations or underwater depths. In Australia, the Australian Height Datum (AHD) is based on mean sea level from 32 tide gauges around the coast.

Global navigation systems

Global positioning system (GPS) and geographic information system (GIS) devices use maps created from geodetic datums. These and other similar terms are in lower case when written out in full: