By the 1990s GPS was a reality for governments, if not individuals, and Australia’s datum and map
grid needed to be made consistent with the rest of the world. So GDA94/MGA94 was adopted. The
origin was at the centre of mass of the earth, hence all coordinates shifted by about 200m, plus the
7cm a year that Australia had moved in that time (about 1.4m). This didn’t officially come into use
until 2000, but the system matched up with the ITRF as at 1 January 1994. As GDA94 is fixed, and
Australia has moved about 1.8m since 1994, this is becoming less accurate compared to GPS where
commercially available devices can now measure to 10cm. In 2000 GPS was only accurate to between
10m and 100m (partly because the US military deliberately included a random error at that time),
hence the shift of a couple of metres was not so important.
The New Systems
To provide a coordinate system that does not detract from the accuracy of the latest GPS devices the
government has now updated the official coordinate systems.
The first update is the datum GDA2020 and map grid MGA2020. This officially began use in 2017 but
was fixed to match the ITRF2014 as at 1 January 2020 – initially it was slightly inaccurate until the
start of this year when Australia moved to the 2020 datum position, and from now on will become
less accurate each year as Australia moves away. The main effect of this is that you should start to
see maps being published using MGA2020 from now on, but the difference to GDA94 will only be a
couple of metres.
For people, such as surveyors, who need the high degree of accuracy the latest GPS devices offer of
10cm, the ATRF2014 has also commenced use as of this year. This is based on the ITRF2014, but we
use more reference points to give higher accuracy for Australia. This will be updated regularly. But
for our purposes in the Pajero Club ATRF2014, ITRF2014 and WGS84 can all be considered the same.
What does this all mean when out on a trip? You need to be careful using AMG66 maps (or just get a
new map as it will be over 20 years old), and if you find your position appears to not match the map
by 200m then be very suspicious that the wrong grid is being used. Otherwise any other datum should
be fine, so long as you’re not trying to land your drone within a metre.
Ian Fieldhouse (#1888) with technical assistance from Neil Phillips (#1744)
References
1. www.icsm.gov.au
2. Geocentric Datum of Australia 2020 Technical Manual, Version 1.3, 21 January 2020,
Intergovernmental Committee on Surveying and Mapping;
3. Geocentric Datum of Australia Technical Manual, Version 2.4, 2 December 2014,
Intergovernmental Committee on Surveying and Mapping;