Datum

Tidal datums are specific tide levels that surveyors (?) use as surfaces of reference for depth measurements in the sea and as a base for determining elevations on land. In leveling operations, the tidal datum most commonly used is the Mean Sea Level.

Surveyors (?) sometimes use other datums, such as mean low water, mean lower low water, mean high water, and mean higher high water, depending upon the purpose of the survey. Still other datums have been used in foreign countries. When conducting leveling operations overseas, check into this matter carefully to avoid mistakes.