Chart datum

From Freepedia

The chart datum is the level of water that charted depths displayed on nautical charts are measured from.

Chart datum is height of water at the lowest spring tide. This is known as the lowest astronomical tide. Lower tides may occur due to meterological effects, such as high pressure systems.

Charted depths and drying heights on nautical charts are given relative to chart datum. Tide tables give the height of the tide above chart datum. This makes it possible to calculate the depth of water at a given point and a given time by adding the charted depth to the height of the tide. To calculate whether an area that dries is under water, subtract the drying height from the height of the tide.