Mapping and coordinate systems are based on a datum, which is a mathematical
surface that best fits the shape of the Earth. Australia's previous datum, the
Australian Geodetic Datum (AGD) was defined in 1966 and best fitted the shape of
the Earth in the Australian region only.
An updated version of this datum,
known as AGD84, was adopted by some Australian States in 1984. AGD84 coordinates
are based on the same datum as AGD66 and for map reading and navigation purposes
can be regarded as being the same.
From the year 2000, all Australian mapping authorities are now using a new
datum, the Geocentric Datum of Australia (GDA), which was defined in 1994.
The primary reason for this change is the widespread use of satellite-based
navigation systems, such as the Global Positioning System (GPS), which is based
on a geocentric datum known as the World Geocentric System 1984 (WGS84). For
most practical purposes, WGS84 and GDA coordinates are the same.
Which datum set do I use?
In all cases the
most important thing to check is that you have the correct map datum set in your
GPS - otherwise you location may be out by up to 250 m. Most modern Australian
maps will be on the GDA94 datum (much the same as the WGS84 datum) although many
Australian topographic maps are still on an old AGD66 datum. The datum should be
listed in the key of the map and you set the appropriate datum in your GPS
What is the difference between datum set ADGA94 and WGS84?
In actual distance there is approximately 10 cm between the two datum set and in
practice there is really no difference for the average user.
Most map coordinates are shown in one of two ways:
given as latitude and longitude values in degrees, minutes and seconds, or
given as easting and northing values, in metres.
Source: This article was derived from a number of sources including
Many thanks to John Louis (csu.edu.au) and Brian Sommerville (GPSOZ.com.au)
answering our questions