Modernising Australia and New Zealand’s national height datums by further refining (quasi)geoid models, assessing user requirements and providing location specific uncertainty.
The Australian Height Datum (AHD) has biases and distortions that introduce discrepancies when heights above mean sea level are determined from measurements made by global navigation satellite system (GNSS) devices. Increasingly, the majority of users access AHD via GNSS positioning with help from AUSGeoid models. Results from a previous CRCSI Project showed the AUSGeoid2020 model has uncertainty up to 0.3 m in densely populated regions.
The Positioning Australia program will provide centimetre-level accurate positioning with mobile devices. When this occurs, the uncertainty of AHD heights accessed via GNSS will be higher than the user’s positional uncertainty throughout much of the country. To ensure users have the capability to perform high accuracy height determination, Australia needs a new national vertical datum that is more compatible with GNSS.
Likewise, the New Zealand Vertical Datum 2016 (NZVD2016) is the second official quasigeoid-based datum for New Zealand. While better than NZVD2009, it does not benefit from the computational improvements developed for the AUSGeoid2020 model. This project assisted in the continued improvement of the NZ vertical datum and quasigeoid model.
The project partners were Geoscience Australia, Curtin University, Land Information New Zealand, Spatial Services NSW, Landgate Western Australia, the Victorian Department of Environment, Land, Water and Planning (DELWP) and the South Australian Department of Infrastructure and Transport (DPTI).
There is a trend internationally towards opting for geoid (equipotential surface of the Earth that corresponds to mean sea level) or quasigeoid (almost a geoid) models to replace levelling-based vertical datums such as AHD. New Zealand and Canada have already adopted this approach and USA is not far behind.
The purpose of this study was to assess the user requirements for height determination, and the technical options which could be implemented to meet the identified user requirements in Australia and New Zealand. The recommendations and outcomes from the research were:
- Adopt a two-frame approach for height in Australia. AHD should remain as the national height datum, and an alternative, the Australian Vertical Working Surface (AVWS), should be made publically available.
- AVWS heights are computed by subtracting Australian Gravimetric Quasigeoid model values from GNSS ellipsoid heights.
- The initial release of the AVWS was realised within the release of the Australian Gravimetric Quasigeoid 2017 (AGQG2017), which has been refined over time as more data has become available. The current version is AGQG2020.
- AGQG models met the key criteria identified in the user requirements study which were:
- Cost-effective to develop and implement.
- Nationally consistent.
- Reliable and robust model to transform heights from GNSS ellipsoidal heights to physical heights with uncertainty over large regions (10s-100s km).
- Compatible with GDA2020, International Terrestrial Reference Frame (ITRF) realisations and Australian Terrestrial Reference Frame (ATRF)
- Easily used to connect onshore and offshore heights.
Offering users access to heights regarding AVWS as well as AHD, provides the best of both worlds. For those whom AHD is fit for purpose, they can continue to use it, whereas for those who it doesn’t serve, can adopt a new modern national height surface (AVWS).
This modernisation of Australia’s height datum is part of ICSM Australian Geospatial Reference System Upgrade Project and Geoscience Australia’s Positioning Australia program. It will improve the capability to perform GNSS height determination efficiently and with confidence. Through user engagement, the project gathered information to make an evidence-based decision to retain AHD and offer an alternative for those who need it.
Clear communication on the differences between derived normal heights from AVWS and AHD heights has been required given that existing AHD benchmarks heights are not the same as normal heights provided with respect to AVWS (having differences of up to approximately 0.5 m). A follow up user feedback survey should be completed to assess changes in user requirements and their opinion of the AVWS.
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