Vertical Datum
D2. Hydrography, tides, waves, bathymetry and marine geologyDefinition
Reference surface for elevations or depths.
A vertical datum is the reference surface against which depths and heights are stated. Nautical charts reduce soundings to a low-water chart datum (lowest astronomical tide in IHO practice, mean lower low water in the United States) so charted depths are conservative; tide tables then add the predicted tide. Ellipsoidal and geoid-based datums tie surveys into a geocentric frame for GPS-RTK hydrography. Datum confusion is a recurring grounding hazard.