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Overview of Australian Height Datum
The national reference system for height is the Australian Height Datum 1971 (AHD71), which is primarily realised through a network of survey control marks.
The Australian Height Datum 1971 (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.
AHD71 is based on mean sea level observation from 1966-1968 at 30 tide gauges around the Australian coast. AHD71 was primarily realised through the national adjustment of traditional levelling survey measurements to derive heights for an integrated network of survey control marks across the country.
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).
The Surveyor-General of Victoria ( SGV) Geodesy division 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 adjusted AHD heights across the state.
Levelling surveys have been performed by surveyors across the state to derive levelled AHD heights on survey marks. Adjusted, levelled and estimated AHD height information is made available for survey marks through the online Survey Marks Enquiry Service (SMES).
AUSGeoid09 and AUSGeoid2020
AUSGeoid09 and AUSGeoide2020 were developed to support determination of AHD height estimates from Global Navigation Satellite Systems (GNSS) positioning. Heights obtained from GNSS positioning are relative to the mathematical reference ellipsoid, and do not refer to AHD. In the case of GDA94 and GDA2020, ellipsoidal heights are related to the Geodetic Reference System 1980 (GRS80) ellipsoid.
AUSGeoid09 and AUSGeoid2020 were created to closely align with adjusted AHD heights on survey control marks. The models feature an additional geometric component which accounts 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).
Using these models, 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.
Please note: There is a scale change between the underlying International Terrestrial Reference Frame that defines GDA94 (ITRF1992) and GDA2020 (ITRF2014). This introduced a 90-100 mm drop in ellipsoidal heights between GDA94 and GDA2020.
- AUSGeoid09 must be used with GDA94 ellipsoidal heights. AUSGeoid09 was developed to provide GNSS users with the height offset between GDA94 ellipsoidal heights and AHD.
- AUSGeoid2020 must be used with GDA2020 ellipsoidal heights. AUSGeoid2020 was developed to provide GNSS users with the height offset between GDA2020 ellipsoidal heights and AHD.
Please refer to Geoscience Australia for more details on the AUSGeoid09 and AUSGeoid2020 models
AHD height from real-time GNSS precise positioning services
The AUSGeoid models are embedded within real-time GNSS CORS network services. The accuracy of AHD heights is dependent on the quality of the GNSS ellipsoidal height and the fit of the AUSGeoid09 or AUSGeoid2020 model to the local AHD network.
Interesting fact sheets related to height variations in Victoria
Ground subsidence monitoring along the Gippsland coastline
In 2019, SGV carried out a GNSS monitoring survey along the Gippsland coast. Results of this survey were compared with a similar survey in 2004 to quantify any deformation across the region. Ground subsidence over the last 15 years was detected at rates of 2 - 6mm per year across the surveyed area.
More information on this campaign is available in the Gippsland subsidence factsheet (DOCX, 1.8 MB).
AHD revision in Morwell
Ongoing subsidence across the Latrobe Valley led to the revision of AHD heights in Morwell and Traralgon.
Updated measurements were included in the Victorian Levelling Network (VLN) adjustment to computed AHD heights that were made available in the SMES database from the end of October 2017.
More information is available in the AHD height revision in Morwell fact sheet . (DOCX, 442.0 KB)
Page last updated: 01/03/21