Objective  The research on topography and landform of the Yangtze estuarine waters is of great significance to human social and economic activities such as river management, river construction and water transportation. Establishing an accurate and seamless chart datum and its transformation model with other vertical datums will directly affect the acquisition of high-precision topographic and geomorphic data at the water-land junctions and the unified comprehensive management and analysis. Thus, this paper studies the establishment of seamless chart datum and its transformation model with other vertical datums for estuarine waters, combining with geoid, sea surface topography and three-dimensional numerical simulation of tidal wave motion.

  Methods  Firstly, the unstructured-grid, finite-volume community ocean model (FVCOM) was used to simulate tidal level and calculate the tidal range at each grid node with the underwater topographic data and the open boundary water level forcing data. According to the correlation between tidal range and chart datum, the chart datum value of each grid node was deduced using tidal range ratios from tide gauge stations with known datums, and a regional seamless chart datum was then established. Secondly, the mean sea surface topography corresponding to a long time scale at each grid node was calculated with the daily sea surface topography product provided by the Copernicus marine environment monitoring service (CMEMS). The mean sea surface topography model was further established by the minimum curvature interpolation. Meanwhile, the geoid in the corresponding area was calculated by the global ultra-high-order gravity field model Earth gravitational model 2008 (EGM2008) with a spatial resolution of 5′ × 5′. Thirdly, the geoid model was established through bi-quadratic interpolation. Finally, according to these three constructed models and the geometric relationship of three kinds of vertical datums in space, unification to a consistent spatial resolution was performed and the separation value between chart datum and the ellipsoid reference datum at each grid node was calculated. The vertical datum transformation model between the seamless chart datum and ellipsoid reference datum was established by the minimum curvature interpolation.

  Results  Using this method, we carried out the experiment at the typical estuarine waters, south branch of the Yangtze Estuary, and established the seamless chart datum and its vertical datum transformation model. The global navigation satellite system (GNSS) leveling data from three long-term tide gauge stations were used for verification and accuracy evaluation of the model. The results show that the absolute errors of the vertical datum transformation model at these three long-term tide gauge stations were 19.8 cm, −25.8 cm and − 27.0 cm respectively, which were all higher than the evaluated error (12.4 cm) of the model. The main reason is that the Yangtze estuarine waters have complicated tidal wave characteristics and they are nearshore waters, which leads to poor accuracy of the mean sea surface topography model in this region, thus further reducing the accuracy of the transformation model as a whole. Despite this, the accuracy of the transformation model still meets the requirements of International Hydrographic Organization standards for hydrographic surveys.

  Conclusions  Therefore, the research method of this paper are feasible and reliable, especially in far- sea areas. When the accuracy of satellite altimetry is further improved in the nearshore areas, the proposed method in this paper can be extended to nearshore and inland waters.