datum is the plane in which we can sketch any thing thing

Land-surface datum is a datum plane that is approximately at land surface at each well. If known, the elevation of the land-surface datum above sea level is given...

Datum: any level surface, line, or point used as a reference in measuring elevations.

According to ASME Y14.5, a primary dimensioning datum requires at least 3 points of contact to establish the datum plane. Two points of contact are required for the secondary datum and 1 point of contact is required for the tertiary datum.

Datum is an imaginary vertical plane from which all horizontal measurements are taken with the aircraft in a level flight attitude.

never below the electrical datum plane.which i believe is the flood level . on a pier or dock at least 12" above the deck but not below the plane . article 555 in the national electric code.