TECHNICAL ARTICLES

APPLICATION OF A GPS-BASED METHOD TO TIDAL DATUM TRANSFER
Dr Chia-Chyang Chang
Department of Surveying and Mapping Engineering, Chung Cheng Institute of Technology, Taiwan
Yuan-Da Sun
Naval Hydrographic and Oceanographic Bureau, Taiwan

Abstract
Observations of the tide are required for the reduction of bathymetric soundings, as each measured depth must be reduced to a tidal datum. If there are no fixed tide gauges located in or near the project area, an auxiliary recording gauge could be installed at a central point and usually operated for 30 days to determine a tidal datum for hydrographic surveys. Alternatively, some computation methods are available to transfer a well-defined datum from fixed tide stations close to the project area. The efforts involved in eliminating such datum errors must be made to ensure the precise bathymetric measurements. Estimates have shown that the datum errors caused by three shortterm tide observations, based on 30, 60 and 90 days, are approximately 10cm in the Taiwan area. Moreover, four traditional computation methods for tidal datum transfer have also been tested to compare the results with those of the GPS-based geometric interpolation method. The results indicated that the GPS-based method is able to provide the most reliable tidal datum, with an average accuracy of around 3cm. This is believed to be sufficient to meet the requirements of a 5cm level of tidal error for Special Order of hydrographic surveys.

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