NOAA's VDatum Tool
The coastal land-water interface depends on how water levels change and the land moves vertically in both space and time. To combine or compare coastal elevations (land heights and water depths) from diverse sources, they must be referenced to the same vertical datum in a common framework. Using inconsistent datums can cause artificial discontinuities that become acutely problematic when producing maps at the accuracy that is critically needed by federal, state, and local authorities to make informed decisions.
VDatum is a free software tool being developed jointly by NOAA's National Geodetic Survey (NGS), Office of Coast Survey (OCS), and Center for Operational Oceanographic Products and Services (CO-OPS). VDatum is designed to vertically transform geospatial data among a variety of tidal, orthometric and ellipsoidal vertical datums - allowing users to convert their data from different horizontal/vertical references into a common system and enabling the fusion of diverse geospatial data in desired reference levels.