AUSGeoid2020 combined gravimetric\u2013geometric model: location-specific uncertainties and baseline-length-dependent error decorrelation

N. J. Brown,A. Woods,I. Baran,W. E. Featherstone,J. C. McCubbine,N. Gowans

doi:10.1007/s00190-018-1202-7

Abstract

AUSGeoid2020 is a combined gravimetric–geometric model (sometimes called a “hybrid quasigeoid model”) that provides the separation between the Geocentric Datum of Australia 2020 (GDA2020) ellipsoid and Australia’s national vertical datum, the Australian Height Datum (AHD). This model is also provided with a location-specific uncertainty propagated from a combination of the levelling, GPS ellipsoidal height and gravimetric quasigeoid data errors via least squares prediction. We present a method for computing the relative uncertainty (i.e. uncertainty of the height between any two points) between AUSGeoid2020-derived AHD heights based on the principle of correlated errors cancelling when used over baselines. Results demonstrate AUSGeoid2020 is more accurate than traditional third-order levelling in Australia at distances beyond 3 km, which is 12 mm of allowable misclosure per square root km of levelling. As part of the above work, we identified an error in the gravimetric quasigeoid in Port Phillip Bay (near Melbourne in SE Australia) coming from altimeter-derived gravity anomalies. This error was patched using alternative altimetry data.

Highlights

Introduction and motivationFitting gravimetric geoid or quasigeoid models to GPSlevelling data is a technique used in many countries
This paper describes the fitting of the Australian Gravimetric Quasigeoid 2017 model (AGQG 2017; Featherstone et al 2018a) to a nationwide GPS-levelling dataset (Featherstone et al 2018b) to provide a model of the separation between the Geocentric Datum of Australia 2020 (GDA2020) ellipsoid and the Australian Height Datum (AHD), enabling a direct transformation between ellipsoidal and AHD heights
AUSGeoid2020 provides the method to convert between GPS-derived ellipsoidal heights and AHD heights and to compute AHD height differences

Summary

Introduction and motivation

Fitting gravimetric geoid or quasigeoid models to GPSlevelling data is a technique used in many countries. GDA2020 is based on the International Terrestrial Reference Frame 2014 (ITRF2014; Altamimi et al 2016) extrapolated to epoch 2020.0 using Australian GPS station velocities (ICSM 2018), under the assumption that the motion is linear This datum change causes horizontal geodetic coordinates to move ~ 1.8 m in a north-easterly direction, primarily due to plate tectonic motion between 1994 and 2020, and ellipsoidal heights to decrease by ~ 0.090 m, due to improvement and refinement of the International Terrestrial Reference System between ITRF92 and ITRF2014. These changes have made it necessary to recompute AUSGeoid because its predecessor was fitted to AHD and GDA94 using LSP (Brown et al 2011). We have devised a decorrelation function (Sect. 4) that can be used to compute the relative uncertainty between GPS-AUSGeoid2020-derived AHD heights, similar to allowable misclosures used in differential levelling

Fitting AGQG2017 to the AHD

Pseudo-independent data

Decorrelation function to compute the relative uncertainty

Conclusion