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Benchmark (surveying) facts for kids

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Benchmark in Edinburgh
An Ordnance Survey cut mark in the UK
Benchmark on rock near Llyn Crafnant
Occasionally a non-vertical face, and a slightly different mark, was used

The term benchmark, bench mark, or survey 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, so that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiseled arrow below the horizontal line.

The term is generally applied to any item used to mark a point as an elevation reference.

Frequently, bronze or aluminum disks are set in stone or concrete, or on rods driven deeply into the earth to provide a stable elevation point.

If an elevation is marked on a map, but there is no physical mark on the ground, it is a spot height.

Purpose

The height of a benchmark is calculated relative to the heights of nearby benchmarks. The position and height of each benchmark is shown on large-scale maps.

Other types of survey marks

Flush bracket G4863
An Ordnance Survey flush bracket

Triangulation points, also known as trig points, are marks with a precisely established horizontal position. These points may be marked by disks similar to benchmark disks, but set horizontally, and are also sometimes used as elevation benchmarks. Prominent features on buildings such as the tip of a church spire or a chimney stack are also used as reference points for triangulation. In the United Kingdom triangulation points are often set in large concrete markers that, as well as functioning as triangulation points, have a benchmark set into the side. 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 three dimensions".

Agencies responsible for benchmarks

Benchmarks are typically placed ("monumented") 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.

Government agencies that place and maintain records of benchmarks include:

Image gallery

  • USCGS-E134

    C&GS benchmark disk in the United States

  • Bench mark at Saint Goussaud, Limousin, France, by Institut Geographique National

    Bench mark at Saint Goussaud, Limousin, France, by Institut Géographique National

  • Torontobenchmark

    City of Toronto benchmark disk in Canada

  • Point Géodesique no. 2378, Terres et Forêts Québec (cropped)

    Benchmark disk in Ottawa, Ontario, Canada

  • Barcdock1

    Vial Moll de Bosch i Alsina, near Port de Barcelona building, & pedestrian crossing Barcelona

  • Charing Cross benchmark

    Benchmark on the plinth of the statue of King Charles I in Trafalgar Square, London: the site of the medieval Charing Cross, and the point from which distances from London are calculated

  • PylonBenchmarkNMKT

    Benchmark painted on an electricity pylon's foundation

Images for kids

  • Dorfkirche Flemsdorf 2020 Vermessungspunkt

    Benchmark of the Prussian Survey Authority on a church in Brandenburg

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