Elevation and Reference

The elevation of any object is its vertical distance above or below an established height on the earth’s surface. This established height is called either a reference plane or a simple reference. The most commonly used reference plane for elevations is mean, or average, sea level, which has been assigned an assumed elevation of 000.0 feet. The reference plane for a construction project is usually the height of some permanent or semi-permanent object in the immediate vicinity, such as the rim of a manhole cover, a rod, or the finish floor of an existing structure. This object may be given its relative sea level elevation, if it is known; or it may be given a convenient, arbitrarily assumed elevation, usually a whole number, such as 100.0 feet. An object of this type, which is used to determine the elevations of other points with a given, known, or assumed elevation, is called a bench mark.