what is benchmark in surveying

In surveying, a benchmark is a post or other permanent mark established at a known elevation that is used as the basis for measuring the elevation of other topographical points. The term benchmark originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future). The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark, which is a point with a precisely known relationship to the vertical datum of the area, typically mean sea level). Benchmarks are typically placed by a government agency or private survey firm, and many governments maintain a register of these marks so that the records are available to all). These records are usually in the form of a geographically searchable database (computer or map-based), with links to sketches, diagrams, photos of the marks, and any other technical details).