Site Selection of Reference Tidal Station for National Height Datum in Taiwan

Abstract

National vertical data are mostly referenced to a tidal level of one or more tidal stations. The Keelung tidal station has been serving as the reference tidal station of the Taiwan National Height Datum for more than one century; all orthometric height in Taiwan is zeroed to the mean sea level (MSL) of an astronomical period of this station. A monument of the Taiwan height origin was established nearby, but due to construction, the height origin was relocated to the newly built National Museum of Marine Science and Technology in Badouzi. This article documents the search for an improved definition of the tidal reference for height origin in Taiwan. The selection criteria identified include geological stability; sufficient depth of water; limited influence from nontidal effects, such as passing vessels; and proximity to the monument of height origin. As part of the preliminary assessment, a tidal station was established at the nearby Cheng-Bin fishery port. The authors performed tidal comparisons of observations collected from the Keelung, Cheng-Bin, and nearby Longdong (to the east) and Taipei Harbor (to the west) stations. In addition to the harmonic analysis, tidal levels of different definitions were computed to determine the potential influence resulting from the change of reference tidal station to the national height datum. From concurrent observations over 3 months, from September 5 to December 17, 2013, the amplitude and phase of significant tidal components were found to be similar between the Keelung and Cheng-Bin stations, whereas the Longdong and Taipei Harbor stations presented larger deviations. The differences between MSL calculations were significant. Although the deviation between the national vertical datum zero and the tidal level was noted in previous studies, the Cheng-Bin station showed relatively smaller changes. Although defining height system with geoid and sea surface topography preserves a more physical and geometric definition of height, a MSL-based definition with a group of coordinated tidal stations may serve as a more reliable scheme for the transitional period.