An artificially established, well surveyed horizontal plane against which elevation, depths, tide, etc. are measured (for example mean sea-level).

A plane of reference for a system of levels which, generally, is taken as zero elevation.

A fixed reference point from which a vertical measurement is taken in recording a specific depth.

A surveyor's plane from which elevations and depths are measured. ebt - A sum of money due by certain and express agreement.

A real or assumed thing, used as a basis for calculations or measurements, as a level from which elevations and depths are measured in surveying.

A horizontal plane used as a reference from which to determine heights or DEPTHS. The plane is called a TIDAL DATUM when defined by a certain phase of the TIDE. Datum planes are referenced to fixed points known as BENCH MARKS, so that they can be recovered when needed. See also REFERENCE PLANE.

A reference plane. A conveniently accessible known surface from which all vertical measurements are taken or referred to.

In layout and machining operations the reference plane from which dimensions are measured in the perpendicular direction.