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- This article is about the use of benchmarks in surveying, for other uses see Benchmark
A benchmark is a point of reference for a measurement. In surveying a benchmark is a geodetic control point set by a surveyor to mark a point of known vertical elevation. These reference points may be chiseled into a wall, or marked by small brass or aluminium disks, concrete posts, iron pins or bolts that are permanently attached to a stable foundation. Such markers are then used as starting points by subsequent surveyors to establish the elevation of nearby points. The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending to a fundamental benchmark, typically a point designated as the mean sea level.
The term benchmark originates from the chiseled horizontal marks that surveyors made into which an angle-iron could be placed to bracket (bench) a levelling rod, thus ensuring that the levelling rod can be accurately repositioned in the same place in the future. These marks were usually highlighted with a chiseled arrow below the horizontal line.
Other types of survey marks
Triangulation points, also known as trig points, are marks used to establish horizontal position. These points may be marked by disks similar to benchmark disks. Often prominent features on buildings such as the tip of a church spire or chimney stack are also used for triangulation. In the United Kingdom triangulation points are often set in small concrete markers, which as well as functioning as a triangulation point, also have a benchmark set into the side of the monument.
Agencies responsible for benchmarks
Benchmarks are typically set ("monumented") by a government agency or a private survey firm.
Government agencies that set and maintain benchmarks include:
- France
- Japan
- United Kingdom
- The Ordnance Survey
- United States
- The National Geodetic Survey (NGS; formerly U.S. Coast & Geodetic Survey)
- The United States Geological Survey (USGS)
See also
- Benchmarking—a recreational activity in which participants search for benchmarks using a handheld GPS receiver.
- Geoid
"Benchmark (surveying)"