Definition - What does Datum Plane mean?
A datum plane is a horizontal or vertical reference point on which surface elevations are based. Though the location of a datum plane is precise, its placement is arbitrary and is determined during the survey stage of a project. Once established, the datum plane will be used for the duration of the project.
Trenchlesspedia explains Datum Plane
Datum planes are given the value of 0. Any distance from the datum plane horizontally or vertically will give a value less than or more than 0 depending on the direction traveled. Establishing a datum plane ensures the project can be completed with as little deviation as possible. Measurements are consistently taken in relation to the datum plane to make sure the operation is following the plans within the allowable tolerances.