Establishing a seamless vertical reference along the tidal segment of the Saint John River

A method is developed to use GPS ellipsoid height observations as a means of applying a seamless vertical reference to hydrographic surveys along a 110 km tidal segment of the Saint John River. In order to be able to use the GPS ellipsoid heights, the Canada national geoid model HTv2.0 is firstly modified using a GPS/levelling geometric method to achieve a locally more accurate transformation from NAD83 ellipsoid height to CGVD28 orthometric height. In the modification, integrating the river shape with the distribution of these GPS/levelling points along the river, few geometric models are tested, and an optimal model, which has about 3 centimetres standard bias in the model interior test and 1 centimetres standard bias in the model exterior test, is chose for the modification of HTv2.0. After the modification of geoid model, the purified HTv2.0 can fulfill the precise transformation from NAD83 ellipsoid heights to CGVD28 orthometric height. The second step is to develop a cross-section linear interpolation method to define an equivalent, continuously varying chart datum reference from which the transformation from CGVD28 height to CD2000 height can be derived. Integrating the definition of chart datum and the linear varying ruler of tide, a new interpolation method is developed. The method can ensure the chart datum definitions of each of points along a cross section are equal, and also guarantee the riverbed topographic continuation and consistency in hydrographic survey after the transformation from CGVD28 height to CD2000 height with the interpolation model. Using this approach to a seamless vertical reference, two on-the-fly GPS tidal measurement experiments are implemented along the tidal segments of Saint John River where the seamless datum reference is constructed. The tidal data processing result with 2 centimeters standard bias and 5 centimeters maximum bias proves that the seamless vertical reference model is credible and can meet with the requirement of hydrographical surveying.