Bench mark is a permanent and recognizable point that lies at a known elevation. A bench mark may be an existing object such as the top of a fire hydrant. In other cases, surveyors bury a concrete post in the ground. A brass plate on top serves as the bench mark.
Surveyors and engineers use bench marks to find the elevation of objects. They also use bench marks to lay out roads, bridges, and other structures at a predetermined elevation. They determine the elevation by reading through a level telescope the distance the point lies above some already established bench mark. The elevation of a new point may be recorded, and the point then becomes another bench mark.
