NOAA VDatum

VDatum converts elevation data among dozens of US vertical datums — orthometric (NAVD 88), ellipsoidal, and the full family of tidal datums (MLLW, MHHW and friends) — as a web tool, desktop application and API. Mixing datums is one of the classic silent errors in coastal work: a metre-scale offset between a lidar DEM on NAVD 88 and a storm-tide boundary on MLLW quietly corrupts every flood elevation downstream.

Any US project where terrain meets tide — coastal flood studies, outfall hydraulics, sea-level-rise screening — runs its data through VDatum at some point. Free, authoritative and maintained by NOAA's National Geodetic Survey and coastal services.