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chart datum
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''〔Australian Bureau of Meteorology National Tide Centre Glossary ''http://www.bom.gov.au/oceanography/projects/ntc/NTC_glossary.pdf'' (retrieved 30 April 2013)〕 and ''mean lower low water''.
A chart datum is a vertical datum and must not be confused with the horizontal datum for the chart.
==Lowest astronomical tide==
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.〔CIRIA (1996). ''Beach management manual''. CIRIA Report 153.〕
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).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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