Preliminary comparisons of the WGS84(EGM 96) geoid with national vertical datums

In fiscal year 2001, the National Imagery and Mapping Agency (NIMA) initiated a project (i) to examine selected national vertical datums in order to evaluate their consistency with orthometric heights derived from Global Positioning System (WGS84) ellipsoid heights and EGM96 geoid heights and (ii) to address the potential for converting bathymetric depths from coastal hydrographic charts to WGS84 depths. This paper discusses preliminary findings obtained during the initial phases of the project. Systematic differences between the EGM96 geoid and various national vertical datums were determined and modeled. These models enable heights to be transformed between local vertical datums and WGS84. A concept for addressing hydrographic chart depth conversion based on GPS positioning of the chart datum (e.g., Mean Low Low Water) is also discussed.