What are the limitations of using World Geodetic System 1984 in Australia?

The World Geodetic System 1984 (WGS84) is the reference frame used by the Global Positioning System (GPS) and developed by the USA Department of Defense (DoD). It is now maintained by the U.S. National Geospatial Intelligence Agency (NGA).

WGS84 has been revised six times since its original realisation in 1987. The most recent version is WGS84 (G2139) implemented on 3 January 2021 and aligned to the International GNSS Service (IGS) realisation of the International Terrestrial Reference Frame 2014, known as IGb14.

The WGS84 reference frame is realised globally through the coordinates of 17 DoD monitoring stations distributed across the world (see figure). WGS84 is a dynamic reference frame. At the beginning of each year, the NGA update station coordinates of the 17 sites. The coordinates are adjusted to an epoch at the half year mark to account for plate tectonic motion. This means that WGS84 coordinates derived by users from GPS measurements will appear to move over time.

There are five main limitations of using WGS84 in Australia that users should be aware of:

  1. There are no official tools for transforming to or from current or legacy Australian geodetic datums to WGS84.
  2. Unlike the Geocentric Datum of Australia 2020 (GDA2020), WGS84 does not have a Recognized-value standard for measurement of position under the National Measurement Act 1960 in Australia.
  3. The accuracy of WGS84 (G2139) as realised using the broadcast ephemeris and ranging data is now typically 2-5 metres.
  4. Australian users are not able to achieve better accuracies as they cannot access WGS84 via a differential measurement directly to the DoD network of stations (see figure).
  5. Since Australia is moving up to 7cm per year, WGS84 coordinates collected 20 years ago will have experienced 1.4 metres of apparent horizontal motion.

Further information:

  • The NGA Standards document NGA.STND.0036_1.0.0_WGS84 - contains a full description of the DoD World Geodetic System 1984 and a summary of its relationship to other geodetic datums.
  • The GPS Standard Positioning Service (SPS) Performance Standard is the document that defines the levels of performance the U.S. government makes available to users of GPS, which is also known as the civilian GPS service.