| Résumé : |
(Auteur) Height systems are usually referred to the mean sea level at tide gauges. Due to the deviation of mean sea level from one equipotential surface of the Earth's gravity field, the geoid, off-sets exist between the various height systems. These off-sets also affect derived quantities such as gravity anomalies. As long as the direct determination of the potential difference between two datum points, by geodetic levelling and gravimetry , is possible the connection, i.e. unification, of height systems is straightforward. It is a serious problem, however, whenever height datum zones are disconnected by sea. With the advent of advanced space positioning techniques and satellite altimetry new solution strategies for the worldwide unification of height systems came into discussion. This investigation deals with the regional unification of height systems for the area of the Indonesian archipelago.
It is a well-known fact that height datum unification is a global problem. However under certain circumstances it is possible to restrict the unification to a certain part of the Earth. Solution strategies make use of a land, a sea and an integrated component. The sea approach is based on satellite altimetry and oceanic levelling, the land approach on space positioning and spirit levelling, including tide gauge registrations. The link between both components is the solution of the geodetic boundary value problem applied regionally, with the aim to provide potential differences between the various vertical datum systems.
The five fundamental elements related to unification of height systems are investigated individually. Mean sea surface heights are determined by satellite radar altimetry, precise orbit ephemerides and corrections for tides and mesoscale ocean variations. By applying collinear pass analysis, crossover adjustment and a least-squares fit to the TOPEX/POSEIDON (T/P) mean sea surface, the mean sea surface in Indonesian waters is determined using the combined altimetry data from the missions Geosat/ERM, ERS-1/35 days, and T/P with an estimated accuracy of 10 cm. The high resolution data set of the ERS-1/168 days repeat mission has been processed with a simplified procedure. The result is consistent with that of the combination solution. As second element sea surface topography is derived from ocean levelling employing hydrographic data. The sea surface topography for Indonesian waters, especially in the eastern Indonesian region, is computed using several oceanographic data sets, the spherical harmonic model of the old Levitus data set up to degree and order 36, data from the Snellius-I and II expeditions, and the recent data set of the World Ocean Atlas 1994. The accuracy of ocean levelling depends on the measurement errors, data sampling and the reference surface. The assumption of a level of no motion at a certain depth, in this investigation 2000 dB is required to eliminate a fundamental indeterminancy. It is a problematic assumption. Further improvement will be possible once better data coverage and higher accuracy measurements are available. To increase the accuracy of local height datum connection, as third element the relative geoid computation is investigated. The relative geoid computation with inclusion of the height off-sets as unknowns should be the appropriate strategy for the solution of height datum connection over small distances between computation points. Finally as fourth and fifth element, the levelling network and the possibility of geodetic space techniques (GPS and VLBI) for height datum connection in Indonesia are reviewed. The levelling networks of Sumatra, Java, Kalimantan, Bali, Lombok, Sulawesi East Timor, Maluku, Seram have been extended by BAKOSURTANAL in recent years. A geodynamic GPS network is under development in the context of the GEODYSSEA project. They offer a promising prospect for use in a precise regional vertical datum connection. From the comparison of sea surface topography as derived from steric levelling and from altimetry and a spherical harmonic geoid model (EGM96) up to degree and order 360, it can be concluded that sea surface topography as computed from the World Ocean Atlas 1994 can be used for preliminary Indonesian height datum connections. The sea surface topography in the South-West coast of Sumatra is about -16 cm, in the South coast of Java it increases from -14 cm (west) to -8 cm (east). |
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