The datum to which the soundings shown on a chart are referred and above which heights of the tide are expressed in tide tables. The level is essentially one below which the sea surface seldom falls. The default tidal datum of the International Hydrographic Organization is the Lowest Astronomical Tide (LAT). The water level will almost never be lower that LAT. Not all hydrographic offices have adopted this datum, but the one used is generally given on the chart.
Subjects: Maritime History.