A bench mark is a permanent, recognizable point that exists at a known elevation (height). It may be an existing object, such as the top of a fire hydrant, or it may be an object specifically placed for measurement purposes, such as a brass plate on top of a concrete post. Surveyors and engineers find the elevation of objects by measuring, through a level telescope, the distance of a point from a preestablished bench mark.
Source: World Book Encyclopedia, vol. 2, p. 247.