Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Land Surveying and Geo-Informatics | en_US |
| dc.contributor.advisor | Tenzer, Robert (LSGI) | en_US |
| dc.creator | Nsiah Ababio, Albertini | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13528 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Modernization of the vertical geodetic datum at the Hong Kong territories | en_US |
| dcterms.abstract | The Hong Kong Principal Datum (HKPD) is the currently adopted official geodetic vertical datum for Hong Kong. The HKPD is practically realized by heights of levelling benchmarks obtained from precise levelling without including gravity measurements along the levelling lines. The HKPD heights are, thus, neither normal nor orthometric. To eliminate systematic errors due to disregarding the gravity information along levelling lines, the gravity information was incorporated in the redefinition and readjustment of the levelling network. The levelling network was readjusted individually for the Molodensky normal and Helmert orthometric heights obtained by applying the normal and orthometric corrections to observed height differences. The results demonstrate the importance of incorporating gravity information even for a relatively small region characterized by a rough topography with heights of levelling benchmarks exceeding several hundreds of meters. The systematic errors due to disregarding the normal and orthometric corrections of levelling segments are minimal. However, their cumulative effect on the benchmarks reaches a high negative value of -4.7 mm for normal heights and 13.8 mm for the corresponding orthometric heights with maximum occurring along levelling lines crossing mountain chains. | en_US |
| dcterms.abstract | The study provides theoretical proof and numerical evidence validating the consistency between the approximate geoid-to-quasigeoid separation and Helmert orthometric heights. It is demonstrated that differences between the Molodensky normal and Helmert orthometric corrections are equivalent to the geoid-to-quasigeoid separation differences computed for individual levelling segments. Furthermore, the accurate geoid-to-quasigeoid separation which considers terrain geometry, topographic density variations, and vertical gravity changes due to mass density heterogeneities below the geoid surface has been computed over the region. Their comparison revealed significant differences which indicates that the geoid-to-quasigeoid separation should be computed accurately, even in regions with moderately elevated topography. A new gravimetric geoid model is also compiled to evaluate its compatibility with the newly defined orthometric heights for Hong Kong. Even though this has not been effectively executed due to the lack of GNSS-levelling benchmarks in the levelling network, the issue of theoretical incompatibility and inconsistencies with orthometric heights has been addressed. | en_US |
| dcterms.abstract | Considering the varied topography and intricate geological setting for a small region like Hong Kong, ignoring the anomalous density can affect the vertical geodetic datum considerably. The magnitude of the lateral topographic density effect increases in vicinities with large topographic variations associated with varied geological formation. Disregarding the actual topographic density in the definition of Helmert orthometric heights can lead to random or systematic errors in the centimeter level or higher depending on the topography and geological setting of the region undergoing vertical datum modernization. The numerical results show that this effect on the geoid and Helmert orthometric heights reach a maxima of about 2.1 and 0.5 cm respectively. This provides clear evidence that regional rock density models are essential in the modernization of vertical geodetic datums especially in regions dominated by sedimentary and igneous rocks despite some criticism that could be raised regarding the reliability of these density models. | en_US |
| dcterms.extent | xxiii, 235 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2024 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Bench-marks -- China -- Hong Kong | en_US |
| dcterms.LCSH | Surveying | en_US |
| dcterms.LCSH | Geomatics | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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