Australian Height Datum

The Australian Height Datum (AHD) is the national vertical datum for Australia. The National Mapping Council adopted the AHD in May 1971 as the datum to which all vertical control mapping would be referred.

This datum surface was created from the simultaneous adjustment of 97,230km of two-way levelling across the country. The adjustment worked from 30 tide gauges distributed around the coast of Australia, where mean sea level was averaged from 1966–1968, and assigned the value of zero on AHD.

Further information on the determination of AHD is given in Division of National Mapping Technical Report No. 12, The Adjustment of the Australian Levelling Survey, 1970–71 (2nd edition, 1975).

The Office of Surveyor-General Victoria (OSGV) manages the network of survey control marks that define AHD in Victoria. This involves maintaining the physical survey control marks as well as levelling observations between those survey control marks that have been collected over the last 50 years. The Victorian levelling observations are combined in one contiguous, state-wide levelling network adjustment dataset, which supports rigorous propagation and determination of AHD across the state.

Level surveys have been performed by surveyors across the state to transfer the adjusted AHD reduced levels on junction points to subsidiary survey marks. AHD values are quoted against survey marks in the online SMES database

AUSGeoid

AUSGeoid09 was developed to support improved determination of AHD height estimates from GNSS observations. Heights obtained from GNSS processing are relative to the mathematical reference ellipsoid, and do not refer to AHD. In the case of GDA, the reference ellipsoid is the Geodetic Reference System 1980 (GRS80) ellipsoid. Thus, AUSGeoid09 was developed to provide GNSS users with the height offset between the GRS80 ellipsoid and AHD.

AUSGeoid09 was created by applying a geometric component to the gravimetric quasigeoid model developed for Australia. This accounted for the offset between the gravimetric quasigeoid and AHD, where the gravimetric quasigeoid is an equipotential surface closely approximated by sea level (approximately 0.5m). AUSGeoid09 AHD-ellipsoid separation values (N-values) can be interpolated for any point in Australia and applied to GNSS derived ellipsoidal heights to derive heights relative to AHD.