Abstract: A two-step transformation,which is from ellipsoidal height to orthometric height then to chart height,is studied.For the first transformation,two kinds of methods are studied.For the step 2,a cross-section linear interpolation method is developed for ensuring these chart datums continuous variation and the transformation from orthometric height to chart height.The above studies are applied for Saint John River,and corresponding models are developed.An on-the-fly GPS tidal level measurement was implemented in the river segment for testing above models.Which proves these methods and theories are credible and feasible in actual applications.