In all maps that are to be contoured some plane, called the datum plane, must be used to which all contours are referred.
A datum plane, or reference surface, from which all elevations are measured up to the ground surface must be assumed.
C, at 100 ft., or greater if necessary to place the datum plane below the ground level at all points within the area to be mapped.
The contours on maps are always numbered, the number of each showing its height above some plane called a datum plane.
Usually the contour has labeled on it in figures the height above some starting point, called the datum plane—generally sea level.
datum plane da·tum plane (dā'təm, dāt'əm, dä'təm)
An arbitrary plane used as a base from which to make craniometric measurements.