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chart datum[′chärt ‚dad·əm]
the level to which sea depth soundings are reduced and from which the depths shown on charts are measured. In the USSR the established chart datum for tideless seas (the Baltic and Black seas, for example) and for lakes is the mean long-term level, and for seas with tides (the Sea of Okhotsk, Bering Sea) it is the lowest sea level, or the theoretical chart datum derived on the basis of sea level observation data. Various standard chart datums used in other countries for seas with tides correspond to the mean spring low-water level, the mean of all low-water levels, and the like.