Tidal Datum
D2. Hydrography, tides, waves, bathymetry and marine geologyDefinition
Reference plane defined by a phase of the tide such as MLLW.
A tidal datum is a reference plane defined by a particular phase of the tide, such as mean lower low water, mean high water, or lowest astronomical tide, computed by averaging tide-gauge observations over a 19-year National Tidal Datum Epoch. Chart datum (a low-water tidal datum) fixes charted depths; high-water datums fix bridge clearances and the legal coastline. Tidal datums vary place to place and must be transferred carefully between gauges.