Benchmark

A benchmark in every-day language is a point of reference for a measurement. In surveying a benchmark is specifically any permanent marker placed by a surveyor with a known vertical elevation (but not neccesarily a known horizontal location). These reference points may be chiseled into a wall, or more typically, marked by small brass or aluminium disks, iron pins or bolts that are permanently attached to a stable foundation, such as concrete posts, bridge abutments, buildings, or a specifically constructed concrete block. These markers are then used as starting (control) points by subsequent surveyors and other users to establish the elevation of nearby points.