![Trigonometrical station](https://upload.wikimedia.org/wikipedia/commons/9/9a/Trigonometrical_station.jpg)
A triangulation station, also known as a trigonometrical point, and sometimes informally as a trig, is a fixed
surveying
Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...
station, used in
geodetic surveying
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure ( geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equiva ...
and other surveying projects in its vicinity. The nomenclature varies regionally: they are generally known as trigonometrical stations or triangulation stations in North America, trig points in the United Kingdom, trig pillars in Ireland, trig stations or trig points in Australia and New Zealand, and trig beacons in South Africa.
Use
The station is usually set up by a government with known
coordinates
In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...
and
elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
published. Many stations are located on hilltops for the purposes of visibility. A graven metal plate on the top of a pillar may provide a mounting point for a
theodolite
A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and i ...
or reflector, often using some form of
kinematic coupling
Kinematic coupling describes fixtures designed to exactly constrain the part in question, providing precision and certainty of location. A canonical example of a kinematic coupling consists of three radial v-grooves in one part that mate with thre ...
to ensure reproducible positioning.
Trigonometrical stations are grouped together to form a network of
triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points.
Applications
In surveying
Specifically in surveying, triangulation involves only angle me ...
. Positions of all land boundaries, roads, railways, bridges and other infrastructure can be accurately located by the network, a task that is essential to the construction of modern infrastructure. Apart from the known stations set up by government, some temporary trigonometrical stations are set up near construction sites for monitoring the precision and progress of construction.
Some trigonometrical stations use the
Global Positioning System
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
for convenience. Its accuracy depends on factors such as ionospheric and tropospheric propagation delay errors.
Although stations are no longer required for many surveying purposes, they remain useful to hikers as navigational aids. Trig points or Triangulation Pillars are another way of spotting the top of a mountain on a map. The symbol for a trig point is a small triangle. They are real concrete pillars that are placed at particular places which are usually the tops of hills or mountains.
Australia
![Geodetic Survey Mark in Wellington](https://upload.wikimedia.org/wikipedia/commons/8/85/Geodetic_Survey_Mark_in_Wellington.JPG)
A national geodetic survey and adjustment carried out in the early 1970s in Australia has left a legacy of trig stations, many consisting of a ground mark with a black quadripod supporting a white square disc above the ground mark.
Hong Kong
Many trigonometrical stations were placed on hilltops around Hong Kong. They strongly resemble those used in other British colonial territories such as Australia, consisting of a white column topped with a black band.
Japan
![3rd class triangulation point in Shiroyama Park in Inagi](https://upload.wikimedia.org/wikipedia/commons/1/18/3rd_class_triangulation_point_in_Shiroyama_Park_in_Inagi.jpg)
In Japan, there are five classes of :
;
:They are installed approximately every , with smaller ones (as necessary) about every .
There are about 1000 throughout Japan. The pillars are on a side, and each pillar is anchored with two very large perpendicular rocks buried underground.
;
:They are installed approximately every . There are about 5000 throughout Japan, and the pillars are on a side. Each pillar is anchored with a very large perpendicular rock buried underground.
;
:There are about 32,000 installed throughout Japan, with one approximately every . The pillars are on a side, and each pillar is anchored with a large perpendicular rock buried underground.
;
:They are installed approximately every , and there are about 69,000 throughout Japan. The pillars are on a side, and each pillar is anchored with a large perpendicular rock buried underground.
;
:These markers were installed in 1899 and are the predecessors to the modern triangulation stations used in Japan today. They are generally not used anymore since the installation of the Class 1-4 stations. Some of them still exist at various locations throughout Japan.
South Africa
![P1000268a The summit of Lion's Head](https://upload.wikimedia.org/wikipedia/commons/5/59/P1000268a_The_summit_of_Lion%27s_Head.jpg)
South Africa has a network of approximately 28,000 trig beacons, established by the
Chief Directorate: National Geo-spatial Information (historically known as the Trigonometrical Survey). These beacons are typically white-painted concrete pillars supporting black metal plates in a cross shape, installed on mountains, hills or tall buildings.
Spain
![ValenciaTorreTriang](https://upload.wikimedia.org/wikipedia/commons/c/c1/ValenciaTorreTriang.JPG)
In Spain there are 11,000 triangulation stations, concrete buildings which typically consist of a cylinder 120 cm high and 30 cm diameter over a concrete cubic base.
They were erected by the
Instituto Geográfico Nacional, usually painted in white, and can be marked with a metallic label with the warning: "The destruction of this sign is punishable by law."
United Kingdom
In the United Kingdom, trig points are typically concrete pillars and were erected by the
Ordnance Survey
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The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurate
retriangulation of Great Britain
__NOTOC__
The retriangulation of Great Britain was a triangulation project carried out between 1935 and 1962 that sought to improve the accuracy of maps made of Great Britain. Data gathered from the retriangulation replaced data gathered during ...
. The
Ordnance Survey
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's first trig point was erected on 18 April 1936 near
Cold Ashby
Cold Ashby is a village and civil parish in West Northamptonshire in England. At the time of the 2001 census, the parish's population was 255 people, increasing to 278 at the 2011 census.
The villages name means 'Ash-tree farm/settlement' or ...
, Northamptonshire. In low-lying or flat areas some trig points are only a few metres above sea level and one is even at −1 m (near
Little Ouse, Cambridgeshire
Little Ouse is a hamlet in Littleport parish, East Cambridgeshire, England, about north-east of Littleport village. It lies on the left bank of the River Little Ouse, which here marks the boundary between Cambridgeshire and Norfolk.
The lowest t ...
, TL61718 89787). When all the trig points were in place, it was possible in clear weather to see at least two other trig points from any one trig point, but subsequent vegetation growth means that this is not necessarily still the case. Careful measurements of the angles between the lines-of-sight of the other trig points then allowed the construction of a system of triangles which could then be referenced back to a single baseline to construct a highly accurate measurement system that covered the entire country.
![Trig point](https://upload.wikimedia.org/wikipedia/commons/d/d0/Trig_point_.jpg)
In most of the UK, trig points are truncated square concrete (occasionally stone)
pyramid
A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilat ...
s or
obelisk
An obelisk (; from grc, ὀβελίσκος ; diminutive of ''obelos'', " spit, nail, pointed pillar") is a tall, four-sided, narrow tapering monument which ends in a pyramid-like shape or pyramidion at the top. Originally constructed by Anc ...
s tapering towards the top. On the top a brass plate with three arms and a central depression is fixed: it is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points. A
benchmark
Benchmark may refer to:
Business and economics
* Benchmarking, evaluating performance within organizations
* Benchmark price
* Benchmark (crude oil), oil-specific practices
Science and technology
* Benchmark (surveying), a point of known elevatio ...
is usually set on the side, marked with the letters "O S B M" (Ordnance Survey Bench Mark) and the reference number of the trig point. Within and below the visible trig point, there are concealed reference marks whose National Grid References are precisely known. The standard trig point design is credited to
Brigadier Martin Hotine (1898–1968), head of the
Trigonometric
Trigonometry () is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. ...
al and
Levelling
Levelling or leveling (American English; American and British English spelling differences#Doubled in British English, see spelling differences) is a branch of surveying, the object of which is to establish or verify or measure the height of sp ...
Division of the Ordnance Survey.
Many of them are now disappearing from the countryside as their function has largely been superseded by
aerial photography
Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other airborne platforms. When taking motion pictures, it is also known as aerial videography.
Platforms for aerial photography include fixed-wing aircra ...
and
digital mapping
Digital mapping (also called digital or computer cartography) is the process by which a collection of spatial data is compiled and formatted into a virtual image on a computer. The primary function of this technology is to produce maps that give ac ...
using
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
s and
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
. To quote from a page at the OS site: "Like an iceberg, there is more of trig pillar below the surface than above it."
OS Blog
accessed 23 Feb 13 From the same source: "Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the entire length of Great Britain."
See also
* Struve Geodetic Arc
The Struve Geodetic Arc is a chain of survey triangulations stretching from Hammerfest in Norway to the Black Sea, through ten countries and over , which yielded the first accurate measurement of a meridian arc.
The chain was established ...
* Bilby tower
A Bilby tower is a type of steel surveying, survey tower used by the United States Coast and Geodetic Survey from 1927 to 1984. It is named after Jasper S. Bilby who designed it in 1926. In 1927, Herbert Hoover, then the Secretary of Commerce, c ...
* Boundary marker
A boundary marker, border marker, boundary stone, or border stone is a robust physical marker that identifies the start of a land boundary or the change in a boundary, especially a change in direction of a boundary. There are several other ty ...
* Geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...
* Ordnance Survey
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, logo = Ordnance Survey 2015 Logo.svg
, logo_width = 240px
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* Retriangulation of Great Britain
__NOTOC__
The retriangulation of Great Britain was a triangulation project carried out between 1935 and 1962 that sought to improve the accuracy of maps made of Great Britain. Data gathered from the retriangulation replaced data gathered during ...
* Survey marker
Survey markers, also called survey marks, survey monuments, or geodetic marks, are objects placed to mark key survey points on the Earth's surface. They are used in geodetic and land surveying. A ''benchmark'' is a type of survey marker that i ...
* Trigpointing
Benchmarking, also known as benchmark hunting, is a hobby activity in which participants find benchmarks (also known as survey markers or geodetic control points). The term "bench mark" is used only to refer to survey markers that designate a ce ...
is a pastime in which people individually go out, find and log the location of trig points.
* Cover of Martyn Bennett
Martyn Bennett (17 February 1971 – 30 January 2005) was a Canadian-Scottish musician who was influential in the evolution of modern Celtic fusion, a blending of traditional Celtic and modern music. He was a piper, violinist, composer and pro ...
's album '' Grit''
References
External links
TrigpointingUK
GPS Waypoints of all UK Trigpoints
Photos of examples of trig points in the UK on geograph.org.uk
Database of trig points in the United Kingdom
Database of trig points in New Zealand
"Trig pillars we salute you", blog post from UK Ordnance Survey
{{DEFAULTSORT:Triangulation Station
Surveying and geodesy markers