The Origin Vertical Shift of National Height Datum 1985 with respect to the Geoidal Surface

The local vertical datum was traditionally defined by the local mean sea level at a tide gauge or at a group of tide gauges. It is realized that the local mean sea level has normally a departure from the geoid possibly up to two meters. In order to evaluate the vertical offset, it is essential to know the geopotential of the mean sea level and of the geoid as well. In this paper we develop models to compute the geopotential of an equipotential surface in terms of the geopotential coefficient model, e.g. EGM96 and DQM99A. We employ the developed models in conjunction with the GPS/leveling data, to calculate the offset of the National Vertical Datum 1985 defined by the mean sea level of Yellow Sea at the Dagang tide gauge in Qingdao city in terms of the 1952～1979 tide gauge records. The results show that the geopotential of the local mean sea level is equal to (62 636 853.40±0.13)m 2s -2 and thus the National Vertical Datum 1985 surface is (0.26±0.05)m above the geoid realized by the geopotential value of W 0=(62 636 856.0±0.5)m 2s -2 .