Definitions for "Chart datum"
The level to which both tidal levels and water depths are reduced - on most UK charts this level is approximately the predicted lowest astronomical tide level (LAT).
The water level used to record data on a chart. Usually the average low tide water level.
The datum to which soundings on a chart are referred. It is usually taken to correspond to a low-water elevation, and its depression below mean sea level is represented by the symbol Z . Since 1989, chart datum has been implemented to mean lower low water for all marine waters of the United States, its territories, Commonwealth of Puerto Rico, and Trust Territory of the Pacific Islands. See datum and National Tidal Datum Convention of 1980.
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