What is GDA?

The Geocentric Datum of Australia (GDA) is a new coordinate system for Australia which is compatible with modern positioning techniques such as the Global Positioning System (GPS). GDA supersedes the existing Australian Geodetic Datum 1966 (AGD66) and Australian Geodetic Datum 1984 (AGD84) coordinate systems. GDA is based on a global framework, the international Terrestrial Reference Frame (ITRF).

What is the difference between the existing AGD and the new GDA coordinates?

The Australian Geodetic Datum (AGD) was established before satellite techniques were available and was based on a model of the Earth, that best suited the Australian region at the time. GDA is based on an international mathematical model which 'best fits' the shape of the whole earth, with its centre coinciding with the earth's centre of mass. Coordinates on the earth's surface change approximately 200 metres in a north easterly direction with the new datum (GDA). The exact change and orientation of the change will vary slightly from area to area.

Q150 Permanent Survey Marks

Click here for a table that shows each of the Permanent Survey Marks in the various formats and forms. All Co-ordinate values are in the same DATUM, being Geodetic Datum of Australia(GDA) for geographic co-ordinates, and Map Grid Australia (MGA) for mappingco-ordinates.

What are geographic co-ordinates and datums (GDA / WGS)?

A geographic co-ordinate system enables every location on the earth to be specified in co-ordinates (latitude, longitude) using a Global Positioning System (GPS). Latitude and Longitude are measured in degrees, minutes and seconds of arc. A datum is a model of the shape of the earth used to help define the geographic co-ordinate system. In Australia the two common datums used today are the Geocentric Datum of Australia (GDA94) and the World Geodetic Datum (WGS84). 
 

What are marine coordinates?

Typically mariners use GPS coordinates based on the World Geodetic Datum (WGS84) and are displayed as degrees and decimal minutes (DD MM.ddd) to 3 decimals (ie. approx 1.8m precision).

To convert from typical land referenced GPS coordinates (DD MM SS.d) to marine GPS coordinates (DD MM.ddd) divide the seconds by 60 (ie. SS.d / 60). This provides the decimals (.ddd) for the minute coordinates. The difference converting from GDA94 to WGS84 datum is ~1m.

See below for more information.

Approximate conversions

• 1 second of arc = 30 metres
• 1 minute of arc = 1800 metres (60 seconds)
• the difference between GDA and WGS datums = 1 metre.
• Co-ordinates based on the old Australian Geodetic Datum (AGD66/84) are 200 metres NE.

What are Cartesian (mapping) co-ordinates (MGA) and projections?
A mapping co-ordinate system specifies each point uniquely using a pair of numerical co-ordinates (easting and northing).  A projection is required to convert spherical earth co-ordinates (latitude and longitude) to mapping co-ordinates. In Australia the current cartesian co-ordinate system is Map Grid Australia (MGA) based typically on the Universal Transverse Mercator (UTM) projection.

For lots more on coordinates, visit the Intergovernmental Committee on Surveying and Mapping site

To learn about the Mathematics behind coordinates, visit Degrees, Minutes, Seconds