Vertical Datums and Their Transformation Approaches for Hydrography

The vertical datums for hydrography and their relations were analysed and the technical approaches for expressing a chart datum on the height datum and Earth ellipsoidal surface were demonstrated. Among the three approaches for vertical datum transportation, developing a set of transportation models recent research has only focused on those relying on the single and discrete tidal station observations and calculations. The accuracy of the mean sea level(MSL) and the chart datum defined with the lowest normal low water was determined while different time spans were estimated with long period tidal station observations. This study indicates that the MSL under 5 cm can be obtained with two years of observations, and a CD under 7 cm root mean squares, along the coast of China. As an example, the CD model over the South China Sea and its separation from the height datum and Earth ellipsoidal surface are established through tide, mean sea surface, and sea surface topography models. Suggestions for unification tidal station CD calculation and establishment of a network for vertical datum control are put forward.