Chart datum

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.^[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. However, it is not 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.^[3]

^ ^a ^b Australian Bureau of Meteorology National Tide Centre Glossary (retrieved 30 April 2013)
^ Sjofartsverket: Mean Sea Level
^ Forrester, W.D. 1983. Canadian Tidal Manual. Chapter 5: Datums and Vertical Control. Department of Fisheries and Oceans, Ottawa, 138pp. [1]

[bom-1] Australian Bureau of Meteorology National Tide Centre Glossary (retrieved 30 April 2013)

[2] Sjofartsverket: Mean Sea Level

[3] Forrester, W.D. 1983. Canadian Tidal Manual. Chapter 5: Datums and Vertical Control. Department of Fisheries and Oceans, Ottawa, 138pp. [1]

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