- This article is about the use of bench marks in surveying, for other uses see Bench mark
A bench mark in every-day language is a point of reference for a measurement. In surveying a bench mark 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 bench mark 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 the future. These marks were usually highlighted with a chiseled arrow below the horizontal
line.
The height of a bench mark is calculated relative to the heights of nearby bench marks in a network extending from a
fundamental bench mark, 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).
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 bench mark disks, but set
horizontally, and sometimes are also used as elevation bench marks. 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 bench mark 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".
Agencies responsible for bench marks
A
C&GS bench mark disk in the US
Bench marks 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 bench marks include:
See also
Wikimedia Commons has media related to:
- Benchmarking—a recreational activity in which participants search for
bench marks using a handheld GPS receiver.
- Geoid
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