On the use of Australian geodetic datums in gravity field determination

The treatment of gravity and terrain data prior to any gravimetric geoid computation is critical. If errors remain in the gravity or terrain data or both, these will propagate into any subsequently determined gravimetric geoid. The effects of horizontal and vertical datums on gravity reduction and, hence, the gravimetric geoid are discussed. Free-air gravity anomalies should be computed on the normal ellipsoid, after a coordinate transformation from the Australian Geodetic Datum, and incorporate a second-order free-air reduction. Their combined effect can reach -0.120mgal or an estimated -12cm in the resulting geoid. Also, the separation between the AHD and the geoid has an effect on the gravimetrically determined geoid. A combined oceanographic and levelling estimate implies that this effect can reach 0.216mgal and 22cm in the geoid. If this rigorous gravity data preparation is employed, centimetric improvements can be expected in all wavelengths of the resulting gravimetric geoid

Satellite and airborne gravimetry: their role in geoid determination and some suggestions

This paper will cover a variety of topics. First, it will briefly overview the GRACE (Gravity Recovery and Climate Experiment) and GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite mission concepts, with a view to the improvements made (and to be made) to the global gravity field. Second, it will summarise some results of the assessment of the recent EGM2008 global gravity field model, which has a spatial resolution of about 10 km. Third, it will describe the computation and evaluation of the AUSGeoid09 model that will be released by Geoscience Australia in the very near future. All three topics will be set in the framework of the restrictions of current data and how airborne gravimetry can contribute. With the increased interest in coastal zone mapping because of threats like sea level change and tsunamis, airborne gravimetry can bridge the gap between land and satellite altimeter-derived gravity data.As such, a proposal will be made to collect airborne gravimetry in key Australian coastal zones, but preferably along the entire coastline! Another area that lacks gravity data is Antarctica, which can adversely affect global gravity field models (the polar-gap problem). Airborne gravimetry has already been used to survey the gravity field of the Arctic, so another proposal will be made to collect airborne gravity over Antarctica. Of course, both are ambitious and massive projects, but it is important to consider them as valuable applications of airborne gravimetry

GNSS-based heighting in Australia: current, emerging and future issues

Ellipsoidal heights, i.e., w.r.t. a geometrical Earth figure, determined from Global Navigation Satellite Systems (GNSS) are inherently their least accurate coordinate, due mainly to satellite geometry and atmospheric refraction. For most practical purposes, however, these GNSS-derived ellipsoidal heights have to be transformed to heights that relate to the Earth’s gravity field, which generally adds further uncertainty. The reduction in accuracy of the transformed height is due to errors in gravimetric quasi/geoid models, but this is compounded yet further in Australia and elsewhere because of the imperfect realisation of local vertical datums. This paper comments upon current, emerging and future issues with height determination on the Australian Height Datum (AHD) using GNSS. This comprises the reference frame used for GNSS ellipsoidal heights, theory- and data-driven inaccuracies in modelling the quasi/geoid, and deficiencies in the realisation of the AHD. While some of these issues will be redressed, in part, by the production of AUSGeoid2008 that is fitted to the AHD, there will always be the need to routinely apply checks on GNSS-derived heights in Australia, and elsewhere

The Importance of Including the Geoid in Terrestrial Survey Data Reduction to the Geocentric Datum of Australia

The complete reduction of terrestrial survey data to the Geodetic Reference System 1980 (GRS80) spheroid will become an important consideration after the implementation of the Geocentric Datum of Australia (GDA). Three examples are used to illustrate that when survey data reduction does not incorporate the effects of the Earth's gravity field, errors of approximately 11ppm, 200m and 3" can be introduced into terrain distances, astrogeodetically determined coordinates and azimuths respectively

Do We Need a Gravimetric Geoid or a Model of the Australian Height Datum to Transform GPS Heights in Australia?

A proposal is made to use a model of the Australian Height Datum (AHD) instead of the classical geoid to provide a more direct transformation of Global Positioning System (GPS) ellipsoidal heights to the AHD. This approach avoids post-survey adjustment of the GPS-AUSGEOID-derived heights in order to align them with existing AHD control. Alternatively, of course, the AHD could be redefined and readjusted such that it is more coincident with the classical geoid, thus allowing the use of a pure gravimetric geoid model in the height transformation. However, the cost and inconvenience associated with implementing a new national vertical datum in the near future render the proposed approach a more practical option in the interim

Only use ship-track gravity data with caution: a case-study around Australia

Much of the ship-track marine gravity data in the Australian national gravity database must not be relied upon because several large (>900 mGal) biases exist in them. These biases were detected and cross-validated through comparisons with marine gravity anomalies derived from re-tracked multi-mission satellite altimetry and a recent satellite-only global geopotential model derived from the Gravity Recovery And Climate Experiment (GRACE). This shows the need to carefully screen ship-track gravity data to ensure that they have been crossover adjusted before they are relied upon in any Earth-science study

A Comparison of Existing Co-ordinate Transformation Models and Parameters in Australia

Four standard procedures to transform curvilinear co-ordinates from the Australian Geodetic Datum 1984 to the World Geodetic System 1984 are compared. These comprise the Bursa-Wolf model with the national set of seven parameters currently used by Federal and State surveying and mapping authorities, the standard Molodensky model with the five parameters used by the United States Defense Mapping Agency, the simple three-parameter model with the origin shifts taken from the Bursa-Wolf and standard Molodensky models, and the multiple regression equations as determined by the Defense Mapping Agency. The differences between the resulting co-ordinates can reach 4.2 metres over continental Australia, which has implications for the final approach adopted to transform to the Geocentric Datum of Australia. The arguments are presented in favour of more suitable transformation strategies using projective transformation models, which are able to simultaneously correct any known errors existing in the Australian Geodetic Datum. These models also allow the direct transformation of both Australian Geodetic Datum 1966 and Australian Geodetic Datum 1984 co-ordinates in a single procedure, which will be of benefit to those States which rely upon older geodetic datums

A comparison of gravimetric geoid models over Western Australia, computed using modified forms of Stokes's Integral

Gravimetric models of the geoid over Western Australia have been constructed using two adapted forms of Stokes's integral; one uses the unmodified Stokes kernel and the other uses a deterministically modified kernel. These solutions use a combination of the complete expansion of the EGM96 global geopotential model with Australian gravity and terrain data. The resulting combined solutions for the geoid are compared with the control given by Global Positioning System (GPS) and Australian Height Datum heights at 63 points over Western Australia. The improved fit of the model that uses a modification to Stokes's kernel indicates that this approach is more appropriate for gravimetric geoid computations over Western Australia

Corrigendum to Featherstone (2006) "Yet more Evidence for a North-South Slope in the Australian Height Datum"

Since the publication of Featherstone (2006), it has been discovered that 339 astrogeodetic vertical deflection stations in Western Australia were omitted from the analysed data set. This was because many Western Australian observations had not been fully incorporated in the astrogeodetic database supplied by Geoscience Australia a few years ago. This corrigendum includes these data into the analysis in Featherstone (2006). While the conclusion that a north-south slope in the AHD remains valid, the inclusion of this additional data makes the evidence for this conclusion less compelling