Benchmark (surveying)

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An Ordnance Survey benchmark in the UK

This article is about the use of benchmarks in surveying, for other uses see 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 precisely known vertical elevation (but not necessarily a precisely 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.

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 levelling rod, thus ensuring that a levelling rod could be accurately repositioned in the same place in the future. These marks were usually highlighted with a chiseled arrow below the horizontal line.

The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark, a point with a precisely known relationship to the level datum of the area, typically mean sea level.

Note that the terms "height" and "elevation" are often used inter-changeably, but in many jurisdictions they have specific meanings - most commonly "height" refers to a local or relative difference in the vertical (such as the height of a building), whereas "elevation" refers to the difference from a nominated reference surface (such as sea-level, or a mathematical/geodetic concept known as the geoid).

[edit] Other types of survey marks

Triangulation points, also known as trig points, are marks with a known horizontal position. These points may be marked by disks similar to benchmark disks, but set horizontally, and sometimes are also used as elevation benchmarks. Often prominent features on buildings such as the tip of a church spire or chimney stack are also used as reference points 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.

With the increasing use of GPS and electronic distance measuring devices, the same techniques and equipment are used to fix the horizontal and vertical position of a survey marker at the same moment, and therefore the marks are usually regarded as "fixed in 3 dimensions".

[edit] Agencies responsible for benchmarks

A C&GS benchmark disk in the US

Benchmarks are typically placed ("monumented") by a government agency or a private survey firm, and many governments maintain a register of these marks so that the records are available to all users. The 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 all the technical details relating to the individual marks.

Government agencies that place and maintain records of benchmarks include: