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Vertical datum

Vertical datum is a reference surface of zero elevation to which heights are referred to. 

In Western Australia the vertical datum consists of the Australian Height Datum 1971 (AHD71), Christmas and Cocos (Keeling) datums which are primarily realised through a network of survey control marks known as benchmarks (accurate vertical information only), and selected standard survey marks with accurate horizontal and vertical coordinates.

NOTE: acronym AHD (Australian Height Datum) often refers to AHD71 as well as Christmas and Cocos (Keeling) datums.


The AHD71 is the official height 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. Further information on the determination of AHD71 is given in Division of National Mapping Technical Report No. 12: The Adjustment of the Australian Levelling Survey, 1970–71 (2nd edition, 1975). Since the introduction of AHD additional levelling surveys have been performed across the state to improve and derive new levelled AHD heights.

Adjusted, levelled and estimated AHD height information is made available through the Map Viewer Plus. For additional information regarding AHD71 modification around Perth please refer to this document. Metro and Buffer Zones can be downloaded  here.

Geoid models

Geoid models allow you to derive AHD heights using Global Navigation Satellite Systems (GNSS) observations without the need to undertake spirit levelling survey. AUSGeoid09 and AUSGeoide2020 were developed to support determination of AHD height using GNSS ellipsoidal height. It is possible because both models feature an additional geometric correction which counts for the offset between the gravimetric quasigeoid and AHD.

AHD-ellipsoid separation values (N-values) can be interpolated for any point in Australia and applied to GNSS ellipsoidal heights to derive heights relative to AHD. For more information as well as tools, models and resources refer to Geoscience Australia website.

NOTE: AUSGeoid2020 is not compatible with GDA94.

Australian Vertical Working Surface

The Australian Vertical Working Surface (AVWS) is a new reference surface for heights in Australia and an alternative to AHD. AVWS is based on Australian Gravimetric Quasigeoid (AGQG) model and can be derived directly from GNSS without needing to connect to any survey mark with accuracy of 4-8 cm across WA.

Geoscience Australia has developed a tool that can transform between ellipsoidal and AWVS heights. To compute AVWS in your software the quasigeoid model is also available for download.