This project continues the “Bathymetric data and Height Datum Analysis” work conducted in 2012. There are two categories of subjects within the scope of this project. The first part is related to the follow-up process of bathymetric data collected in the airborne bathymetric lidar mapping project. The area covers Dongsha atoll and a portion of Penghu islands. This year, the contour layer of 1 m interval is checked and revised. The second category is the height system. The current height systems include orthometric height, ellipsoid height, and tidal based height. Related to the ellipsoid height, the stability of eight framework satellite tracking stations is analyzed. For the tidal, three tidal stations, one in Dongsha and two in Keelung area, are maintained and data analyzed. For the satellite tracking station analysis, there are two items. The first is the stability analysis of the tracking station based on the archived observations. The second is the relative stability between the main station and the auxiliary monuments. In this year, regarding the long-term analysis of satellite tracking stations, the velocity field of tracking stations are generally toward north and east, and the trends of velocity field magnitude are decreased from north to south of Taiwan. The velocity field of TNSM, KMNM and MZUM are approximately 3cm/yr toward north and 1 cm/yr toward west. For the relative stability, new coordinate changes found significant based on the survey conducted.
In Dongsha tidal analysis, the mean sea levels computed by arithmetic mean and harmonic analysis of observations are -0.0561m and -0.0527m. The DEM of orthometric height and 1m contours in Dongsha area are re-produced. In Keelung tidal analysis of Zhang-Tan-Li tidal station, the mean sea levels computed by arithmetic mean and harmonic analysis of observations are 0.0881m and 0.0996m from ultra-sonic tide gauge, 0.0396m and 0.0389m from pressure tide gauge, and 0.0843m and 0.0846m from radar tide gauge. In tidal analysis of Cheng-Bin tidal station, the mean sea levels computed by arithmetic mean and harmonic analysis of observations are -0.0533m and -0.0426m.
Based on the study up to this year, the maintainness of the Dongsha tidal station and the two tidal stations in Keelung area is highly recommended. This shall be able to provide the long lasting operation of national height system valuable contribution. Further evaluation on the implementation of ellipsoid height system is another highly recommended subject. Due to its economic value and high reliability in terms of international connection, the importance of ellipsoidal height is increasing. While tidal based height is indispensable for navigation and charting, a seamless height datum is also important. On this regard, the currently available global tide models such as DTU10 and TCXO 7.2 could serve as a good reference for high spatial resolution local model.