Will Featherstone

Comparison and validation of recent freely-available ASTER-GDEM ver1, SRTM ver4.1 and GEODATA DEM-9S ver3 digital elevation models over Australia

This study investigates the quality (in terms of elevation accuracy and systematic errors) of three recent publicly available elevation model datasets over Australia: (i) the 9 arc second national GEODATA DEM-9S ver3 from Geoscience Australia and the Australian National University; (ii) the 3 arc second SRTM ver4.1 from CGIAR-CSI; and (iii) the 1 arc second ASTER-GDEM ver1 from NASA/METI. The main features of these datasets are reported from a geodetic point of view. Comparison at about 1 billion locations identifies artefacts (e.g. residual cloud patterns and stripe effects) in ASTER. For DEM-9S, the comparisons against the space-collected SRTM and ASTER models demonstrate that signal omission (due to the ∼270 m spacing) may cause errors of the order of 100–200 m in some rugged areas of Australia. Based on a set of geodetic ground control points over Western Australia, the vertical accuracy of DEM-9S is ∼9 m, SRTM ∼6 m and ASTER ∼15 m. However, these values vary as a function of the terrain type and shape. Thus, CGIAR-CSI SRTM ver4.1 may represent a viable alternative to DEM-9S for some applications. While ASTER GDEM has an unprecedented horizontal resolution of ∼30 m, systematic errors present in this research-grade version of the ASTER GDEM ver1 will impede its immediate use for some applications.

Absolute and relative testing of gravimetric geoid models using Global Positioning System and orthometric height data

The verification of gravimetric geoid models on land is presented using co-located Global Positioning System (GPS) and orthometric heights. This is undertaken in an absolute sense, which also constrains the zero-degree term in the geoid, and in a relative sense, which indicates the appropriateness of the gravimetric geoid model for transforming GPS-derived ellipsoidal heights to the local vertical datum. Two computer programs are presented for these purposes, which can also be used for comparing other types of discrete and gridded data. A case study is used to illustrate the use of these approaches and software to validate gravimetric geoid models in Western Australia.

Height systems and vertical datums: A review in the Australian context

This paper reviews (without equations) the various definitions of height systems and vertical geodetic datum surfaces, together with their practical realisation for users in Australia. Excluding geopotential numbers, a height system is a one‐dimensional coordinate system used to express the metric distance (height) of a point from some reference surface. Its definition varies according to the reference surface chosen and the path along which the height is measured. A vertical geodetic datum is the practical realisation of a height system and its reference surface for users, nominally tied to mean sea level. In Australia, the normal‐orthometric height system is used, which is embedded in the Australian Height Datum (AHD). The AHD was realised by the adjustment of ∼195,000 km of spirit‐levelling observations fixed to limited‐term observations of mean sea level at multiple tide‐gauges. The paper ends by giving some explanation of the problems with the AHD and of the differences between the AHD and the national geoid model, pointing out that it is preferable to recompute the AHD.

A geodetic approach to gravity data reduction for geophysics

The currently adopted approach to reduce observed gravity data for geophysical purposes includes several approximations. These were originally used to reduce computational eAort, but have remained standard practice, even though the required computing power is now readily available. In contrast, more precise gravity reductions are routinely employed in physical geodesy. The diAerence between simple Bouguer gravity anomalies derived using the geophysical and geodetic approaches can reach several tens of mm sec ˇ2 . The geodetic reductions include a more accurate calculation of normal gravity as a function of latitude, and a free air correction that accounts for the non-sphericity of the figure of the Earth. Also important, especially given the advent of Global Positioning System coordi- nation of gravity surveys, is the need to ensure that the correct vertical and horizontal coordinate sys- tems are used for the gravity reduction procedure. Errors associated with the use of non-geocentric horizontal coordinates and ellipsoidal heights are significant when compared with the accuracy of an in- dividual gravity measurement. A generalised gravity reduction program and a coordinate transform- ation program are presented which can be employed to reduce geophysical data in a geodetic manner. # 1998 Elsevier Science Ltd

Complete spherical Bouguer gravity anomalies over Australia

Complete (or refined) spherical Bouguer gravity anomalies have been computed for all 1 095 065 land gravity observations in the June 2007 release of the Australian national gravity database. The spherical Bouguer shell contribution was computed using the supplied ground elevations of the gravity observations. The spherical terrain corrections, residual to each Bouguer shell, were computed on a 9 arc-second grid (∼250 m by ∼250 m spatial resolution) from a global Newtonian integration using heights from version 2.1 of the GEODATA digital elevation model (DEM) over Australia and the GLOBE and JGP95E global DEMs outside Australia. A constant topographic mass-density of 2670 kg/m3 was used for both the spherical Bouguer shell and spherical terrain correction terms. The difference between the complete spherical and complete planar Bouguer gravity anomaly exhibits an almost constant bias of about −18.7 mGal over areas with moderate elevation changes, thus verifying the planar model as a reasonable approximation in these areas. However, the results suggest that in mountainous areas with large elevation changes, the complete spherical Bouguer gravity anomaly should be selected in preference over the less-rigorous complete planar counterpart.

How Well Can Online GPS PPP Post-processing Services be Used to Establish Geodetic Survey Control Networks?

Abstract Establishing geodetic control networks for subsequent surveys can be a costly business, even when using GPS. Multiple stations should be occupied simultaneously and post-processed with scientific software. However, the free availability of online GPS precise point positioning (PPP) post-processing services offer the opportunity to establish a whole geodetic control network with just one dual-frequency receiver and one field crew. To test this idea, we compared coordinates from a moderate-sized (~550 km by ~440 km) geodetic network of 46 points over part of south-western Western Australia, which were processed both with the Bernese v5 scientific software and with the CSRS (Canadian Spatial Reference System) PPP free online service. After rejection of five stations where the antenna type was not recognised by CSRS, the PPP solutions agreed on average with the Bernese solutions to 3.3 mm in east, 4.8 mm in north and 11.8 mm in height. The average standard deviations of the Bernese solutions were 1.0 mm in east, 1.2 mm in north and 6.2 mm in height, whereas for CSRS they were 3.9 mm in east, 1.9 mm in north and 7.8 mm in height, reflecting the inherently lower precision of PPP. However, at the 99% confidence level, only one CSRS solution was statistically different to the Bernese solution in the north component, due to a data interruption at that site. Nevertheless, PPP can still be used to establish geodetic survey control, albeit with a slightly lower quality because of the larger standard deviations. This approach may be of particular benefit in developing countries or remote regions, where geodetic infrastructure is sparse and would not normally be established without this approach.

EVIDENCE OF A NORTH-SOUTH TREND BETWEEN AUSGeoid98 AND THE AUSTRALIAN HEIGHT DATUM IN SOUTHWEST AUSTRALIA

Abstract The AUSGeoid98 gravimetric geoid model has been compared with 48 GPS-levelling points at a ∼50 km spacing across part of the southwest of Western Australia. This is arguably the best subset of GPS-derived ellipsoidal heights in Australia with an internally estimated precision of <±9 mm. The sprit-levelled heights were tied to the Australian Height Datum (AHD) using class C techniques [12mm-root-km allowable misclose]. The comparisons show that AUSGeoid98 gives a GPS height transformation to the AHD with a precision of ∼±13 cm, which is less than reported earlier (∼±36 cm) for a nationwide dataset. A clear north-south trend of ∼O.81 mm/km [ppm] is also evident in the differences; of which approximately one-third is attributable to a north-south error in the AHD induced by dominant north-south sea surface topography effects at the nearby fixed tide gauges. After removal of this north-south trend, the standard deviation of the differences reduces to ∼5 cm.

Comparison of digital elevation models over Australia and external validation using ERS-1 satellite radar altimetry

Digital elevation models (DEMs) are widely relied upon as representations of the Earths topographic morphology. The most widely used global DEMs available are ETOPO5, TerrainBase and JGP95E at a 5‐arc‐minute spatial resolution, and the GTOPO30 and GLOBE (version 1) global DEMs at a 30‐arc‐second spatial resolution. This paper presents the results of intercomparisons of these global DEMs over Australia, and with the GEODATA 9‐arc‐second DEM (version 1) of Australia. These DEMs were also compared to an independently produced, altimeter‐derived orthometric height database. This allows not only a totally independent assessment of the quality of these different DEMs over Australia, but also an insight into the ERS‐1 radar altimeters ability to measure orthometric heights on land. The results of all these comparisons reveal large differences among the DEMs, with the greatest difference between JGP95E and ETOPO5 (mean 49 m, standard deviation ±274 m). The comparison with the altimeter‐derived database shows good agreement with the version 1 GEODATA DEM (mean 2 m, standard deviation ±27 m), thus demonstrating that the altimeter is a viable method for quality assessment of DEMs in lowland regions. A further conclusion is that the representation of the Australian land surface in both the JGP95E and TerrainBase global DEMs is more accurate than the higher resolution GLOBE (version 1) global DEM, even though JGP95E displays a disparity along the 140°E meridian because of the different data sources used in its construction.

GRACE Hydrological Monitoring of Australia: Current Limitations and Future Prospects

The Gravity Recovery and Climate Experiment (GRACE) twin‐satellite gravimetry mission has been monitoring time‐varying changes of the Earths gravitational field on a near‐global scale since 2002. One of the environmentally important signals to be detected is temporal variations induced by changes in the distribution of terrestrial water storage (i.e., hydrology). Since water is one of Australias precious resources, it is logical to monitor its distribution, and GRACE offers one such opportunity. The second and fourth releases (referred to as RL02 and RL04) of the ‘standard’ monthly GRACE solutions with respect to their annual mean are analysed. When compared to rainfall data over the same time period, GRACE is shown to detect hydrological signals over Australia, with the RL04 data showing better results. However, the relatively small hydrological signal typical for much of Australia is obscured by deficiencies in the standard GRACE data processing and filtering methods. Spectral leakage of oceanic mass changes also still contaminates the small hydrological signals typical over land. It is therefore recommended that Australia‐focussed reprocessing of GRACE data is needed for useful hydrological signals to be extracted. Naturally, this will have to be verified by independent ‘in situ’ external sources such as rainfall, soil moisture and groundwater borehole piezometer data over Australia.

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.