Australia has used three different geodetic datums:
� AGD66 – Australian Geodetic Datum effective 6 October 1966;
� GDA94 – Geocentric Datum of Australia effective 1 January 1994 (in use from 2000);
� GDA2020 – Geocentric Datum of Australia effective as at 1 January 2020 (in use from 2017);
From the geodetic datum a map grid is created by projecting the coordinate system (which is
spherical) onto a flat surface. The most common projection used is the Universal Trans Mercator
projection or UTM.
We have had three map grids:
�
�
�
AMG66 – Australian Map Grid 1966 - map grid based on AGD66;
MGA94 – Map Grid of Australia 1994 – map grid based on GDA94;
MGA2020 – Map Grid of Australia 2020 – map grid based on GDA2020
The above map grids are fixed to the continent at a certain point in time.
So MGA94 is the coordinate system fixed in 1994. This means the
coordinates of a place in Australia will not change in that system at any
point in time. A time dependent reference frame however will change the
coordinates over time. Picture the latitude and longitude frame fixed to the
centre of the Earth, but set just above the Earth’s surface, kind of like a
cage. As Australia moves (at 7cm per year) across the Earth’s surface and
under the frame, the coordinates of a place in Australia will change in a
time dependent reference frame.
Figure 2: Latitude & Longitude
This is what is occurring with GPS coordinates. This means that in addition to the normal coordinates
you need to specify the time of the system – Eg 2020.0 for the start of 2020 or 2020.5 for 30 June
2020. WGS84, the US system is the best known example of this type of system, but WGS and the
international version called ITRF are essentially the same thing as they match at centimetre level.
WGS84 (and therefore GPS coordinates) is updated twice a year to account for the continental shift.
Time dependent reference frames include:
� ATRF2014 – Australian Terrestrial Reference Frame 2014
� ITRF2014 – International Terrestrial Reference Frame 2014
� WGS84 – World Geodetic System 1984
Effect of GPS and Australia Moving
In the 1960s Australia for the first time adopted a consistent datum and
map grid. Because GPS wasn’t all that relevant then, the shape that was
used for the earth (the geoid) was chosen to best fit Australia, rather than
the whole world. Unfortunately, this meant the origin or centre of the
coordinate system was about 200m from the centre of mass of the Earth.
The difference is because the material in the Earth’s mantle and core are
not regularly distributed. The reference point for this system was the
Johnston Geodetic Station which is about 7km north of the SA-NT border
just off the Stuart Highway.
Figure 3: Stone Marker Johnston Geodetic Station