Friday, March 12, 2010

AUSGeoid09: Converting GPS heights to AHD heights

Australian Surveying and Land Information Group (AUSLIG) (now Geoscience Australia) introduced new version of gravimetric geoid model, AUSGeoid09. AUSGeoid09 is still undergoing final testing before its release in mid-2010. It allows Global Positioning System (GPS) users to convert between GPS heights and Australian Height Datum (AHD) heights. In Australia, heights above mean sea level (MSL) are referenced to the Australian Height Datum (AHD). It was established by setting the mean value observed at 32 tide gauges around Australia between 1966 and 1968 to a height of 0.000 metres. Levelling techniques were then used to transfer heights relative to MSL across Australia.

GPS receivers are now widely used for accurate positioning and navigation in Australia. They use a different reference surface known as the ellipsoid. It is a simplified approximation of the Earth that looks like a basketball which has been slightly squashed at the top and bottom. Unfortunately, ellipsoid is not directly compatible with the AHD. Consequently, there has been an increasing demand for a method of combining the speed of GPS data acquisition and the practicality of the AHD. Hence, AUSGeoid09 is the answer. It offers significant productivity gains for GPS users by allowing them to compute AHD heights either in the field in real time or back in the office.

AUSGeoid09 is a three dimensional model used to convert ellipsoidal heights (as observed by GPS) to AHD heights to within ±0.050 metres accuracy across most of Australia. Older versions of AUSGeoid are predominantly based on satellite and terrestrial gravity observations which were a best fit of The Geoid over Australia (Featherstone et al 2001). These AUSGeoid versions, referred to as gravimetric geoids, do not account for the one metre offset trend between The Geoid and the AHD. As a result, when using these versions GPS users can only retrieve AHD heights to within ±0.5 metres.

But, AUSGeoid09 is slightly different. Instead of only using gravity data, it also includes a geometric component developed from GPS and AHD data which describes the approximate one metre offset trend between the AHD and the gravimetric geoid. The gravimetric and geometric components are combined together into a single national grid with two kilometre resolution. The addition of the geometric component means that the AUSGeoid09 model is no longer a true representation of the Geoid because it is not an equipotential surface. However, the AUSGeoid name is retained for familiarity.

Check it here.


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