Gravity

Gravity field and height determination

We have been active in Australian and international geoid determination (the equipotential surface of the Earth’s gravity field), funded throughout by numerous Australian Research Council grants. Our research includes a wide range of theoretical and methodological issues, coupled with software development.

In 1998, we provided the methods and software to produce the AUSGeoid98 quasigeoid model, which remains the national standard for the transformation of Global Positioning System (GPS) heights to the Australian Height Datum. A technical description of AUSGeoid98 is provided and the model can be downloaded free of charge from Geoscience Australia.

We are currently producing a new Australian quasigeoid model that will supersede the previous AUSGeoid98 national standard model for GPS heighting.

Global and regional gravimetric geoid modelling

  • Claessens, S.J., W.E. Featherstone and I.M. Anjasmara (in press) Is Australian data really validating EGM2008, or is EGM2008 just in/validating Australian data? In: Mertikas, S. (ed.) Gravity Geoid and Space Missions, Springer, Berlin Heidelberg New York
  • Abeyratne, P.G.V. and W.E. Featherstone (2009) Assessment of EGM2008 over Sri Lanka, an area where ‘fill-in’ data were used in EGM2008, Newton’s Bulletin 4: 284-316.
  • Claessens, S.J., W.E. Featherstone, I.M. Mujitsarama and M.S. Filmer (2009) Is Australian data really validating EGM2008, or is EGM2008just in/validating Australian data? Newton’s Bulletin 4: 207-251.
  • Deng, X.L., R. Coleman, W.E. Featherstone and K.R. Ridgway (2009) Assessment of geoid models offshore Western Australia using in-situ measurements, Journal of Coastal Research 25(3): 581–588, doi: 10.2112/07-0972.1.
  • Featherstone, W.E. (2009) Only use ship-track gravity data with caution: a case-study around Australia, Australian Journal of Earth Sciences 56(2): 191-195, doi: 10.1080/08120090802547025.
  • Featherstone, W.E. and D.D. Lichti (2008) Fitting gravimetric geoid models to vertical deflections, Journal of Geodesy 83(6): 583-589, doi: 10.1007/s00190-008-0263-4
  • Morgan, P.J. and W.E. Featherstone (2009) Evaluating EGM2008 over East Antarctica, Newton’s Bulletin 4: 317-331.
  • Ebner, R. and W.E. Featherstone (2008) How well can online GPS PPP post-processing services be used to establish geodetic survey control networks? Journal of Applied Geodesy 2(3): 149-157, doi: 10.1515/JAG.2008.017.
  • The University Component of the AuScope Geospatial Team (2008) New geodetic infrastructure for Australia, Journal of Spatial Science 53(2): 65-80.
  • Featherstone, W.E. (2007) Augmentation of AUSGeoid98 with GRACE satellite gravity data, Journal of Spatial Science 52(2): 75-86.
  • Featherstone, W.E. and L. Morgan (2007) Validation of the AUSGeoid98 model in Western Australia using historic astrogeodetically observed deviations of the vertical, Journal of the Royal Society of Western Australia 90(3): 143-149.

Synthetic Earth gravity models

Two different synthetic/simulated Earth gravity models (SEGMs) have been developed so far at Curtin: a global gravity model (CurtinSEGM), and a regional model over Australia only (AusSEGM).  Both synthetic models provide a realistic representation of the Earth’s gravity field, so are well suited to validating, and thus possibly improving, the procedures and software currently in place for gravity field determination and modelling. AusSEGM is now being used to test geoid determination techniques in Australia and Canada. A new global forward gravity model is under construction, which will allow modelling of the gravity field inside the topographic masses, and will be used to validate different height systems.

  • Kuhn, M., W.E. Featherstone and J.F. Kirby (2009) Complete spherical Bouguer gravity anomalies over Australia, Australian Journal of Earth Sciences 56(2): 209-219, doi: 10.1080/0812009080254704.
  • Tsoulis, D. and M. Kuhn (2007) Recent developments in synthetic Earth gravity models in view of the availability of digital terrain and crustal databases of global coverage and increased resolution, Proceedings of the 1st International Symposium of the International Gravity Field Service “Gravity Field of the Earth”, Istanbul, Turkey.

Ellipsoidal physical geodesy

For over 300 years, geodesists have known that the Earth’s figure is more like an oblate ellipsoid of revolution than a sphere. However, physical geodesists often still use spherical Earth models and approximations. Therefore, we are examining – from first principles – the complete treatment of the Earth’s gravity field in a purely ellipsoidal framework, ranging from satellite-based gravity field determination to regional geoid modelling. This will lead to a general theory of ellipsoidal physical geodesy, which will improve the accuracy of gravity field and geoid modelling and thus allow us to fully profit from data observed by the new dedicated satellite gravity missions. Recent work in this direction has generalised the Meissl spectral scheme for the geodetic ellipsoid.

  • Claessens, S.J. and W.E. Featherstone (2008) The Meissl scheme for the geodetic ellipsoid, Journal of Geodesy 82(8): 513-522, doi: 10.1007/s00190-007-0200-y
  • Featherstone, W.E. and S.J. Claessens (2008) Closed-form transformations between geodetic and ellipsoidal coordinates, Studia Geophysica et Geodaetica 52(1): 1-18, doi: 10.1007/s11200-008-0002-6

Forward gravity field modelling

We have developed two comprehensive software packages to estimate the gravitational effects of global geo-referenced mass distributions (e.g., topographic or crustal masses) using space- and spectral-domain techniques. The latter software provides approximate results, but is well suited to modelling global mass distributions in a time-efficient way. The former software directly evaluates Newton’s integral in the space domain using discretised numerical integration based on the superposition of the gravitational effect of regular shaped bodies (e.g., rectangular or spherical prisms). These approaches can be applied to large datasets on a standard PC. The software has recently been applied to compute spherical terrain corrections over Australia at a 9-arc-second (200m) spatial resolution.

Non-stationary least-squares collocation

Least-squares collocation is an optimal interpolation and prediction tool for gravity field modelling, somewhat akin to Kriging in geostatistics. We have used least-squares collocation for merging of ship-track and satellite altimeter gravity data, which was used in AUSGeoid98, the national standard geoid model. We also devised and implemented a cross-validation technique that gives a more objective indication of the correlation length and noise of the covariance function when merging GPS-levelling and gravimetric geoid data. We are currently implementing anisotropy and non-stationarity into standard least-squares collocation, which will ultimately enable it to optimally interpolate and predict spatially variable gravity field data.

GNSS height determination

The National Collaborative Research Infrastructure Strategy (NCRIS), through AuScope Geospatial , will establish over 100 CORS (continuously operating GNSS – global navigation satellite system – reference stations) across Australia. Approximately one third of the $65 million AuScope budget is allocated to install and maintain a CORS network. Through the Cooperative Research Centre for Spatial Information, we are determining the optimal way to select the best location and distribution of CORS sites to support as-many-as-possible scientific and commercial users of the new geodetic network.

  • Featherstone, W.E. (2008) GNSS-heighting in Australia: current, emerging and future issues, Journal of Spatial Science 53(2): 115-133

Assessment and unification of heights

For several years, we have investigated the integrity of the Australian Height Datum, which forms the fundamental vertical spatial data infrastructure in Australia. Current work is redefining this height datum using more sophisticated modelling, processing and adjustment strategies. We have also devised a rigorous orthometric height system that can be embedded in this new vertical datum. In collaboration with Land Information New Zealand, we have implemented a geoid-based vertical datum in New Zealand that also unifies the 13 separate vertical datums in use there. We ultimately aim to make vertical datums fully compatible with regional geoid models, thus allowing direct height determination from GPS.

  • Filmer, M.S., M. Kuhn and W.E. Featherstone (in press) Correction to Angus-Leppan, P.V. (1979) Refraction in levelling – its variation with ground slope and meteorological conditions, Journal of Spatial Science
  • Amos, M.J. and W.E. Featherstone (2009) Unification of New Zealand’s local vertical datums: iterative gravimetric quasigeoid computations, Journal of Geodesy 83(1): 57-68, doi: 10.1007/s00190-008-0232-y
  • Filmer, M.S. and W.E. Featherstone (2009) Detecting spirit-levelling errors in the AHD: recent findings and some issues for any new Australian height datum, Australian Journal of Earth Sciences 56(4): 559-569, doi: 10.1080/08120090902806305
  • Featherstone, W.E. and M.S. Filmer (2008) A new GPS-based evaluation of distortions in the Australian Height Datum in Western Australia, Journal of the Royal Society of Western Australia 91(2): 199-206
  • Featherstone, W.E. (2007) Corrigendum to “Yet more evidence for a north-south slope in the Australian Height Datum” Journal of Spatial Science 52(1): 65-68.

Non-stationary least-squares collocation

Least-squares collocation is an optimal interpolation and prediction tool for gravity field modelling, somewhat akin to Kriging in geostatistics. We have used least-squares collocation for merging of ship-track and satellite altimeter gravity data, which was used in AUSGeoid98, the national standard geoid model. We also devised and implemented a cross-validation technique that gives a more objective indication of the correlation length and noise of the covariance function when merging GPS-levelling and gravimetric geoid data. We are currently implementing anisotropy and non-stationarity into standard least-squares collocation, which will ultimately enable it to optimally interpolate and predict spatially variable gravity field data.