Science / Geology / Datum: A reference location or elevation which is used as a starting point for subsequent measurements. Sea level is a datum for elevation measurements. Datums can also be arbitrary such as the starting point for stream stage measurements or based upon a physical feature such as the base of a rock unit.
Search Google for Datum:
Science / Tides and Currents / Chart Datum: 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 1980, c MORE
Science / Tides and Currents / Sea Level Datum (SLD): An obsolete term. See National Geodetic Vertical Datum of 1929 and mean sea level. MORE
Science / Tides and Currents / Gulf Coast Low Water Datum (GCL WD): A tidal datum. Used as chart datum from November 14, 1977, to November 27, 1980, for the coastal waters of the Gulf coast of the United States. GCLWD is defined as mean lower low water when the type o MORE
Science / Tides and Currents / Geodetic Datum: See National Geodetic Vertical Datum of 1929 (NGVD 1929) and North American Vertical Datum of 1988 (NAVD 1988). MORE
Science / Tides and Currents / North American Vertical Datum: A fixed reference for elevations determined by geodetic leveling. The datum was derived from a general adjustment of the first-order terrestrial leveling nets of the United States, Canada, and Mexico. MORE