A New GPS-based evaluation of distortions in the Australian Height Datum in Western Australia

Previous work on assessing the errors in the Australian Height Datum (AHD) across Western Australia used fewer and older global positioning system (GPS) data and a global quasigeoid model. A larger and improved State-wide set of 243 GPS-derived ellipsoidal heights and a regional gravimetric quasigeoid model are now available. Therefore, it is possible to re-evaluate the north-south tilt in the AHD and look for regional systematic distortions with some more confidence in Western Australia. This new analysis shows an apparent north-south tilt of ~0.27 mm/km in the existing AHD over the whole of the State, but which increases to ~0.6 mm/km over smaller regions, showing regional systematic distortions. When mean sea-level constraints are removed from the AHD by a minimally constrained least-squares adjustment of the spirit-levelling observations that is less prone to the effect of sea-surface topography, the north-south tilt reduces to ~0.18 mm/km, but the regional distortions remain, showing that errors are present in the spirit-levelling observations

Error propagation for three common height-system corrections to differential levelling

This paper investigates the propagation of input data errors through the application of Helmert orthometric, normal and normal-orthometric height corrections to differential levelling observations, these being the three principal height systems in practical use around the world. Height corrections are required to remove the systematic error resulting from the geometric non-parallelism of the Earth's equipotential surfaces, but different height systems propagate errors differently. These systematic errors are thus present within levelling networks and subsequently in local vertical datums. Here, we show that the Helmert orthometric correction is sensitive to errors in the mean value of gravity along the plumbline, particularly for heights above 1000 m. The normal correction is much less sensitive due to the use of normal gravity along the normal plumbline. The normal orthometric correction of Rapp (1961) is largely insensitive to such errors, but it does not properly correct for the non-parallelism of the Earth's equipotential surfaces. Information showing the circumstances under which survey practitioners should apply height corrections to levelling lines is provided, demonstrating that normal-orthometric corrections only need be applied to class LC levelling lines that are to be used for large levelling networks extending in the north-south direction, particularly at high elevations

An investigation into the displacement of Permanent Survey Marks in the Hillcrest area resulting from reactive soils

Reactive soils in the Adelaide suburb of Hillcrest (South Australia) have resulted in concrete Permanent Survey Marks (PSMs) being horizontally displaced. This has been identified by different surveys over the past 50 years showing differences in relative measurement between PSMs. It has been assumed that this movement relates directly to the seasonal wetting and drying of reactive soils found in the area. A monitoring project was established, which found that minimal movement occurred within the 10 month study period. The results suggest that any substantial horizontal displacement previously identified is a gradual movement occurring over a number of years rather than seasonally

Three viable options for a new Australian vertical datum

While the Intergovernmental Committee on Surveying and Mapping (ICSM) has stated that the Australian Height Datum (AHD) will remain Australia’s official vertical datum for the short to medium term, the AHD contains deficiencies that make it unsuitable in the longer term. We present and discuss three different options for defining a new Australian vertical datum (AVD), with a view to encouraging discussion into the development of a medium- to long-term replacement for the AHD. These options are a (1) levelling-only, (2) combined, and (3) geoid-only vertical datum. All have advantages and disadvantages, but are dependent on availability of and improvements to the different data sets required. A levelling-only vertical datum is the traditional method, although we recommend the use of a sea surface topography (SSTop) model to allow the vertical datum to be constrained at multiple tide-gauges as an improvement over the AHD. This concept is extended in a combined vertical datum, where heights derived from GNSS ellipsoidal heights and a gravimetric quasi/geoid model (GNSS-geoid) at discrete points are also used to constrain the vertical datum over the continent, in addition to mean sea level and SSTop constraints at tide-gauges. However, both options are ultimately restricted by the requirement to upgrade the Australian National Levelling Network (ANLN). It is also desirable that the ANLN be kept in reasonable shape for the validation or testing of height products.On the other hand, a geoid-only vertical datum, where GNSS-geoid is used to continuously define the vertical datum, has advantages primarily because it avoids the requirement to level long distances to upgrade the levelling network. However, it is not routinely possible to realise a geoid of the desired 1–2 cm accuracy necessary to develop a geoid-only vertical datum, especially to a local precision that can match levelling, such that a geoid-only vertical datum is considered a long-term proposition. In the meantime, a combined vertical datum is a more suitable option for any new AVD in the next decade or so, although a geoid-based vertical datum which retains only the higher-quality parts of the ANLN in Australia's densely settled areas, but connected by a geoid model rather than continent-wide levelling, may also have merit

Detecting spirit-levelling errors in the AHD: recent findings and issues for any new Australian height datum

The Australian Height Datum (AHD) forms the vertical geodetic datum for Australia and is thus the framework for all heights, including those used to establish digital elevation models (DEMs). The AHD was established over quite a short timeframe, due to the urgent requirement for height control for topographic mapping and gravity surveys. This necessitated the use of lower quality spirit-levelling observations over long distances and approximate data reductions. Geoscience Australia has kindly supplied us with height differences for all sections of the basic and supplementary spirit-levelling used to establish the AHD, allowing us to analyse loop closures to detect spirit-levelling (or data entry / transcription) errors in this dataset. In the case-studies presented here, we show that GPS and a precise gravimetric quasigeoid model can be used to identify the sections in a levelling loop that cause misclosure, reflecting the relative quality of modern quasigeoid models over the spirit-levelling originally used to establish the AHD. We also consider and discuss some of the other issues that would have to be considered if Australia is to implement a new vertical geodetic datum from these data to support, for example, improved DEMs in the future

A re-evaluation of the offset in the Australian Height Datum between mainland Australia and Tasmania

The adoption of local mean sea level (MSL) at multiple tide-gauges as a zero reference level for the Australian Height Datum (AHD) has resulted in a spatially variable offset between the geoid and the AHD. This is caused primarily by sea surface topography (SSTop), which has also resulted in the AHD on the mainland being offset vertically from the AHD on the island of Tasmania. Errors in MSL observations at the 32 tide-gauges used in the AHD and the temporal bias caused by MSL observations over different time epochs also contribute to the offset, which previous studies estimate to be between ~+100 mm and ~+400 mm (AHD on the mainland above the AHD on Tasmania). This study uses five SSTop models (SSTMs), as well as GNSS and two gravimetric quasigeoid models, at tide-gauges/tide-gauge benchmarks to re-estimate the AHD offset, with the re-evaluated offset between −61 mm and +48 mm. Adopting the more reliable CARS2006 oceanographic-only SSTM, the offset is −12 ± 11 mm, an order of magnitude less than three previous studies that used geodetic data alone. This suggests that oceanographically derived SSTMs should be considered as a viable alternative to geodetic-only techniques when attempting to unify local vertical datums

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

Is Australian data really validating EGM2008, or is EGM2008 just in/validating Australia data?

The tide-free release of the EGM2008 combined global geopotential model and its tide-free pre-release PGM2007A are compared with Australian land, marine and airborne gravity observations, co-located GPS-levelling on the [admittedly problematic] Australian Height Datum, astrogeodetic deflections of the vertical, and the AUSGeoid98 regional gravimetric quasigeoid model. In all comparisons, EGM2008 performs better than any previous global gravity model. The standard deviation of the differences between free-air gravity anomalies from EGM2008 and free-air gravity anomalies from Australian land gravity observations is 5.5 mGal, compared to, e.g., 11.7 mGal for EGM96. Furthermore, the standard deviation of the differences between height anomalies from EGM2008 and anation-wide set of 254 GPS-levelling points is 17.3 cm, compared to, e.g., 33.4 cmfor EGM96. In the comparisons with GPS-levelling, EGM2008 also outperforms AUSGeoid98 (standard deviation of 19.1 cm in the differences with the nation-wide set of 254 GPS-levelling points), and the same holds for the comparison to astrogeodetic deflections of the vertical. However, due to the poor quality of some of the Australian data, we cannot legitimately claim to truly validate EGM2008. Instead, EGM2008 confirms the already-known problems with the Australian data, as well as revealing some previously unknown problems. If one wants to claim validation, then EGM2008 is validated implicitly because it can confirm the errors in our regional data. Simply, EGM2008 is a good model over Australia

Nonlinear subsidence at Fremantle, a long-recording tide gauge in the Southern Hemisphere

© 2015 The Authors. A combination of independent evidence (continuous GPS, repeat geodetic leveling, groundwater abstraction, satellite altimetry, and tide gauge (TG) records) shows that the long-recording Fremantle TG has been subsiding in a nonlinear way since the mid-1970s due to time-variable groundwater abstraction. The vertical land motion (VLM) rates vary from approximately -2 to -4 mm/yr (i.e., subsidence), thus producing a small apparent acceleration in mean sea level computed from the Fremantle TG records. We exemplify that GPS-derived VLM must be geodetically connected to the TG to eliminate the commonly used assumption that there is no differential VLM when the GPS is not colocated with the TG. In the Perth Basin, we show that groundwater abstraction can be used as a diagnostic tool for identifying nonlinear VLM that is not evident in GPS time series alone. Key Points: The Fremantle tide gauge is and has been subsiding in a nonlinear way Exemplar of the need for geodetic connection between tide gauge and GPS station Groundwater has been used as a diagnostic for nonlinear vertical land movement