Chart datum

Chart datum is the theoretical level of water in any tidal area during the lowest possible astronomical tide (LAT). Lower tides may occur due to meterological effects

Depths (or drying heights) on nautical charts are given relative to chart datum. Tide tables give the depth of water above chart datum, making it possible to calculate the depth of water at a given point by adding the depth (or subtracting the drying height) to (from) the depth given in the tide table.