Comparison of national vertical and chart datums with WGS 84 (EGM96) geoid

In 2001, the National Imagery and Mapping Agency (NIMA) initiated a project with the Naval Surface Warfare Center (i) to examine transformations from national vertical data to the World Geodetic System 1984 and (ii) to address the potential for converting bathymetric depths from coastal hydrographic charts to WGS 84. This paper discusses findings obtained during the project. Systematic differences between various national vertical data and WGS 84 were determined and modeled. These models enable elevations or orthometric heights to be transformed with a better "fit" between local vertical data and the WGS 84 geoid. A concept for addressing hydrographic chart depth conversion based on GPS positioning of the chart data (e.g. Mean Low Low Water) was also examined. Results along the southeastern United States coast are presented.