博士 / 國立交通大學 / 土木工程學系 / 101 / This study determines the vertical datum differences between Taiwan and its offshore islands, i.e., Liouciou, Lyudao, Lanyu and Penghu, based on a geodetic theory that converts potential differences to datum height differences. The local vertical datums (LVDs) of Taiwan and its offshore islands are the local mean sea levels derived from tide gauge records. The differences between the datums are the vertical datum offsets, which are closely related to the sea surface topography (SST) values that can also be derived by an ocean model. To determine the LVD offsets, determinations of ellipsoidal heights by GPS and geoidal undulations at points belonging to two different vertical datums are needed. For a precise geoid modeling, dense and evenly distributed high precision marine gravity data are needed. However, neither existing shipborne data nor altimeter-derived data can meet this requirement. Airborne gravity data can provide dense gravity coverage, but the corresponding spatial resolution is not competitive with that of shipborne data due to the high speed of the aircraft carrying out the airborne measurements. For the conversion of gravity values from one altitude to another, a 3-D Fourier series is used to fit the observed gravity anomalies at the flight altitude for downward continuation to the sea level or a desired altitude.
From 2006 to 2010, small ships that are able to sail to the immediate coastal areas were used for the first time to collect shipborne gravity data around Taiwan's offshore islands. A popular procedure of airborne gravity data processing is used to process the offshore shipborne gravity data with some modifications. The Bernese 5.0 software is used for relative kinematic positioning of ships. For many existing ship gravity datasets, ship positions and velocities derived from GPS navigation solutions are used for gravity reductions, resulting in positioning-induced errors. Such errors are evaluated in this study. By assessing filter performances using spectrum analyses, an optimal low-pass Gaussian filter is designed to suppress the high frequency noises in ship gravity data. The quality of the offshore shipborne gravity data is assessed using crossover analysis, yielding RMS crossover differences of less than 2 mgal in all cases. Compared to the altimeter-derived DTU10 gravity model, the offshore shipborne gravity data reveal a detailed change of bathymetry. The maximum difference between the DTU10 and shipborne gravity values near the coast of Lanyu reaches 30 mgal.
The gravimetric geoid modeling is based on the remove-compute-restore procedure, with the reference field being the EGM08 model. The residual geoid is computed by Stokes’ integral with a modified kernel to eliminate the long wavelength errors in the gravity data. The accuracy of the resulting geoid model is assessed using geoidal undulations at leveling benchmarks derived from ellipsoidal heights (GPS) and orthometric heights (precision leveling). The standard deviations of the differences between the gravimetric and GPS-derived geoid undulations in flat areas are mostly below 5 cm, and are about 1 cm along the coastal area of Keelung. An iterative method of geoid modeling is used that takes into account the LVD offsets in gravity reductions. With the gravimetric geoid model and 48-hour GPS observations collected at the vertical datum connection sites, the vertical datum differences with respect to the Keelung mean sea level (TWVD2001) are 24.6 2.1, 50.4 9.3, 105.8 2.2, and 44.1 2.4 cm for Liouciou, Lyudao, Lanyu and Penghu, respectively.
| Identifer | oai:union.ndltd.org:TW/101NCTU5015036 |
| Date | January 2013 |
| Creators | Huang, Chi-Hsun, 黃啟訓 |
| Contributors | Hwang, Chein-Way, 黃金維 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 132 |
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