Datum, Plane
Datum, Plane : The horizontal plane to which soundings, ground elevations, or water surface elevations are referred. Also: Reference Plane. The plane is called a Tidal Datum when defined by a certain phase of the tide. The following datums are ordinarily used on hydrographic charts: MEAN LOW WATER--Atlantic Coast (U. S.), Argentina, Sweden, and Norway. MEAN LOWER LOW WATER--Pacific Coast (U. S MEAN LOW WATER SPRINGS--United Kingdom, Germany, Italy, Brazil, and Chile. LOW WATER DATUM--Great Lakes (U. S. and Canada). LOWEST LOW WATER SPRINGS--Portugal. LOW WATER INDIAN SPRINGS--India and Japan. See: INDIAN TIDE PLANE. LOWEST LOW WATER--France, Spain, and Greece. A common datum used on United States topographic maps is Mean Sea Level. See also: Bench Mark
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