Mean lower low water

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

A chart datum is a vertical datum and must not be confused with the horizontal datum for the chart.

Many national charting agencies, including the United Kingdom Hydrographic Office and the Australian Hydrographic Service, use the Lowest Astronomical Tide (LAT) - the height of the water at the lowest possible theoretical tide - to define chart datums. LAT is the lowest levels which can be predicted to occur under average meteorological conditions.

One advantage of using LAT is that all predicted 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).

The United States' National Oceanic and Atmospheric Administration uses mean lower low water (MLLW), which is the average height of the lowest tide recorded at a tide station each day during the recording period (the National Tidal Datum Epoch - a 19 year period). MLLW is only a mean, so some tidal levels may be negative relative to MLLW, see also Mean low water spring.

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 Springs or Highest Astronomical Tide (for "HAT" see "tidal range").

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