Australian Datums (and how to move between them)

This diagram shows how users can move between the range of legacy Australian datums and the new datums released in the Upgrade to the Australian Geospatial Reference System

GDA94 and GDA2020 (Static Datums)

  • GDA2020 is a static datum – just like GDA94
  • A ‘static’ datum means that the coordinates of features (e.g. roads, buildings and property boundaries), do not change with time
  • GDA94 - coordinates of features are based on the position of the Australian plate in 1994
  • GDA2020 - coordinates of features are based on the position of the Australian plate in 2020 (~1.8 m northeast of GDA94).
  • The upgrade to GDA2020 is to ensure spatial data can be more closely aligned to positions observed using Global Navigation Satellite System (e.g. GPS)
  • GDA2020 is available for use now
  • Users can transform between GDA94 and GDA2020 using transformation parameters or grids.

ATRF

  • The Australian Terrestrial Reference Frame (ATRF) is a time dependent reference frame – just like the International Terrestrial Reference Frame (ITRF)
  • A ‘time-dependent reference frame / earth fixed’ means that positions of features change with time due to plate tectonic motion
  • ATRF will be available for use from 1 January 2020.
  • GDA2020 and ATRF provide a “two-frame” approach to support different user requirements. This means, both GDA2020 and ATRF will be legal datums as of 1 January 2020
  • Coordinates can be propagated between GDA2020 and ATRF using the Australian Plate Motion model.

AVWS

  • The Australian Vertical Working Surface (AVWS) is a new reference surface for heights in Australia
  • AVWS is a gravity model that provides the offset between the ellipsoid and geoid
  • The AUSGeoid model provides the offset between the ellipsoid and Australian Height Datum (AHD)
  • A height user requirements study found the Australian Height Datum (AHD) and AUSGeoid are not capable of meeting some user requirements
  • AVWS heights can be computed directly from GNSS without needing to connect to survey mark infrastructure with accuracy of 1-8 cm
  • AUSGeoid2020 has an accuracy of 6-13 cm
  • AHD will remain as the only legal height datum
  • AVWS will be available for use from 1 January 2020.