The terms benchmark , or bench marks , are derived from carved horizontal markings made by surveyors in stone structures, where the elbow can be placed to form a "bench" for the measuring rod , thus ensuring that the regulator rod can be accurately repositioned in the same place in the future. These marks are usually indicated with a chisel arrow below the horizontal line.
This term is generally applied to each item used to mark a point as an elevation reference. Often, bronze or aluminum disks are placed on rocks or concrete, or on shafts that are pushed deep into the earth to provide a stable elevation point. If the elevation is marked on the map, but there is no physical mark on the ground, it is the height of the place.
The benchmark height is calculated relative to the height of the nearest benchmark in a network extending from the fundamental benchmark. The fundamental benchmark is a point with a known exact relationship to the datum level of the area, usually meaning sea level. The position and altitude of each benchmark are shown on a large-scale map.
The terms "high" and "elevation" are often used interchangeably, but in many jurisdictions they have a special meaning; "height" usually refers to local or relative in vertical difference (such as building height), whereas "elevation" refers to the difference from the nominated reference surface (such as sea level, or the mathematical/geodetic model approaching the ocean surface known as the geoid). The altitude can be determined as a normal height (above the reference ellipsoid), orthometric heights, or dynamic altitudes that have slightly different definitions.
Video Benchmark (surveying)
Other types of survey signs
The triangulation points, also known as trigonometric points, are marks with precisely defined horizontal positions. These points can be marked by a disk similar to a benchmark disk, but horizontally arranged, and sometimes also used as a height benchmark. Prominent features in buildings such as the tip of the church tower or chimney are also used as reference points for triangulation. In the UK the triangulation points are often arranged in large concrete markers which, as well as functioning as triangulation points, have the bench set to the side. With the increasing use of GPS and electronic distance measuring devices, the same techniques and equipments are used to improve the horizontal and vertical position of the survey markers at the same time, and therefore the mark is usually regarded as "fixed in three dimensions".
Maps Benchmark (surveying)
Agency responsible for benchmark
Benchmarks are usually placed ("monumented") by government agencies or private survey companies, and many governments maintain this list of signs so records are available to all. These recordings are usually in the form of geographically searchable databases (computer or map based), with links to sketches, diagrams, sign photos, and other technical details.
Government agencies that place and maintain benchmarking records include:
- Canada
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- Canada Natural Resources (Geodetic Survey Division)
- France
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- National GÃÆ'Â © ographique Institute (IGN in FR FR).
- India
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- India Survey
- Italy
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- Istituto Geografico Militare
- Japan
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- Geographical Survey Institution (GSI)
- New Zealand
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- New Zealand Land Information
- Pakistan
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- Pakistan Survey
- South Africa
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- Rural Development Department and Land Update
- Spanish
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- Instituto GeogrÃÆ'¡fico Nacional (IGN)
- United Kingdom
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- Ordnance Survey
- United States
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- National Geographic Geodetic Survey (NGS; US Coastal & Geodetic Survey)
- United States Army Engineer Corps (USACE)
- United States Forest Service
- United States Geological Survey (USGS)
Image gallery
Also see
- Benchmarking - recreational activities where participants are gaining benchmarks using handheld GPS receivers.
- Arrow width
- Geoid
- Leveling - a survey technique that uses benchmarking
- Altitude of places
- The survey marker - an object placed to mark the main survey points
External links
- Canada's Spacial Reference System
- History of the Canadian Geodetic Survey Division
- Istituto Geografico Militare - Servizio Geodetico
- Pakistan Survey
- UK database benchmark
- Detailed description of different types of UK benchmarks
- The US Benchmark NGS is overlaid on Google Map
- Rankings and pointing points
Source of the article : Wikipedia
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