Charts and Tables
Charted depths and drying heights on a nautical charts are given relative to chart datum. Some height values on charts, such as vertical clearances under bridges or overhead wires, may be referenced to a different vertical datum, such as mean high water.
Tide tables give the height of the tide above a 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.
Using charts and tables that are not based on the same datum can result in incorrect calculation of water depths and should be avoided. Check that they are compatible before use.
Read more about this topic: Chart Datum
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