Datum Any numerical or geometric quantity or set of such quantities that may serve as a reference or base for other, comparable quantities. For example, Mean Sea Level (MSL) is the datum used on most topographic maps. However, most river gages use an arbitrary elevation above the National Geodetic Vertical Datum (NGVD) of 1929 for use as a zero datum (e.g., datum equals 3412.6 feet above NGVD of 1929). Datums are always chosen so there will never be negative stages.
Source: http://www.bvsde.paho.org/bvsacg/i/fulltext/dicciona/dicciona.pdf
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Author of the Water Words Dictionary source of text: Gary A. Horton
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