Definition
Chart datum is the vertical reference for soundings and drying heights, chosen so that the tide will seldom fall below it. In the United Kingdom and most IHO member states it is set at or near Lowest Astronomical Tide, so predicted heights of tide are added to charted depths to obtain actual depth. It is a local datum specific to each port or region, not a continuous surface, and it differs from the datums used for land heights.
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