chart datum

Article summary

A chart datum is the water surface serving as origin (or coordinate surface) 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. Common chart datums are lowest astronomical tide (LAT) and mean lower low water (MLLW). In non-tidal areas, e.g., the Baltic Sea, mean sea level (MSL) is used. A chart datum is a type of vertical datum and must not be confused with the horizontal datum for the chart. However, it is necessarily an equigeopotential (a water "level surface"): the chart datum is tilted across smaller to larger tidal range regions; in rivers, it is a sloping and undulating surface following the low stage.

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