Chart datum
Level of water from which depths displayed on a nautical chart are measured / From Wikipedia, the free encyclopedia
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A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum.[1] Common chart datums are lowest astronomical tide (LAT)[1] and mean lower low water (MLLW). In non-tidal areas, e.g. the Baltic Sea, mean sea level (MSL) is used.[2] A chart datum is a type of vertical datum and must not be confused with the horizontal datum for the chart.