From Wikipedia, the free encyclopedia

An Ordnance Survey bench mark in the UK

This article is about the use of bench marks in surveying, for other uses see Bench mark

In everyday language, a benchmark is a point of reference for any measurement. In surveying a benchmark is specifically a survey marker with a precisely known vertical elevation. On a specific construction project, surveyors will use temporary benchmarks, typically a wooden post hammered into the ground. More enduring benchmarks may be chiseled into a wall (the usual type in the UK) or marked by a small brass or aluminium disk, steel pin, or bolt attached to a stable foundation, such as a concrete post, bridge abutment, building, or a purpose-made concrete block. These markers are then used as starting (control) points for subsequent surveys 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 leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in future. These marks were usually indicated 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. The position and height of each benchmark is shown on large-scale maps.

Note that the terms "height" and "elevation" are often used interchangeably, but in many jurisdictions they have specific meanings; "height" commonly 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).

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, which as well as functioning as a triangulation point, 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".

A C&GS benchmark disk in the US

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:

• France
  • Institut Géographique National (IGN on Wiki FR)
• Japan
  • Geographical Survey Institute (GSI)
• United Kingdom
  • The Ordnance Survey
• United States
  • The National Geodetic Survey (NGS; formerly U.S. Coast & Geodetic Survey)
  • The United States Geological Survey (USGS)
• New Zealand
  • Land Information New Zealand]

Wikimedia Commons has media related to:

Trig points and benchmarks

• Benchmarking—a recreational activity in which participants search for benchmarks using a handheld GPS receiver.
• Geoid

UK benchmark database

Detailed description of the different types of UK benchmark