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The chart datum is the level of water that charted depths displayed on nautical charts are measured from. The chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water.
Lowest Astronomical Tide
Many national charting agencies, including the United Kingdom Hydrographic Office and other hydrographic services, such as those of Canada and Australia, that originated with the British Admiralty use the Lowest astronomical tide (LAT), the height of the water at the lowest possible theoretical tide, as chart datum. The advantage of using LAT is that all tidal heights must then be positive (or zero) avoiding possible ambiguity and the need to explicitly state sign. Calculation of the LAT only allows for gravitational effects so lower tides may occur in practice due to other factors (e.g. meteorological effects such as high pressure systems).
Mean Lower Low Water
The United States' National Oceanic and Atmospheric Administration uses mean lower low water (MLLW), which is the average of the lowest tide recorded at a tide station each day during the recording period. MLLW is generally located above LAT and therefore some tidal states may have negative heights.
Charts and Tables
Charted depths and drying heights on 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 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 based on a different datum will result in incorrect calculation of water depths and must be avoided. Check that they are compatible before use.