To describe property located above or below the earth’s surface, such as the air rights of a condominium, a surveyor must know the property’s elevation. Standard elevation reference points, called datums, have been established throughout the country. The original datum was defined by the U.S. Geological Survey as mean sea level at New York harbor. A surveyor uses a datum as an official elevation point to describe the height or depth of a property. If, for example, the datum for an area is a point 100 feet above sea level, all surveys in the area will indicate elevation as a distance above or below 100 feet above sea level.

In many cases it is impractical for a surveyor to rely on a single datum for an entire surveying area. To simplify matters, surveyors have identified local elevation markers, called benchmarks, to provide reference elevations for nearby properties. Once a benchmark is registered, it provides a valid reference point for surveying other elevations in the immediate area.