1. 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. 2. The latitude and longitude of an initial point; the azimuth of a line from this point. 3. The top or bottom of a bed of rock on which structure contours are drawn.
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