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Atriangulation station, also known as atrigonometrical point, and sometimes informally as atrig, is a fixedsurveying station, used ingeodetic surveying and other surveying projects in its vicinity.
The station is usually set up by a mapping organisation with knowncoordinates andelevation published. Numerous stations are installed onsummits for purposes of visibility and prominance. A graven metal plate on the top of a pillar may provide a mounting point for atheodolite or reflector, often using some form ofkinematic coupling to ensure reproducible positioning.
Trigonometrical stations form networks oftriangulation. Positions of all land boundaries, roads, railways, bridges and other infrastructure can be accurately located by the network, a task 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 theGlobal Positioning System for convenience. Its accuracy factors in ionospheric and tropospheric propagation delay errors.
Although stations are no longer required for many modern surveying purposes, they remain useful to hikers and evenaviators as navigational aids. Particular small triangles on maps (◬) mark summits, for example.
The nomenclature for triangulation stations varies regionally: they are generally known astrigonometrical stations ortriangulation stations in North America,trig points in the United Kingdom,trig pillars in Ireland,trig stations ortrig points in Australia and New Zealand,[1] andtrig beacons in South Africa.
In the 1820s, much of New South Wales was unsurveyed territory to the European arrivals. To aid the mapping of the country, the science of trigonometic surveying was introduced by Major Thomas Mitchell who had been brought out to the colony as Assistant Surveyor General of New South Wales. The freestanding peak of Mount Jellore was selected as the first trigonometric summit for his triangulation survey of the countryside. In 1828 Mitchell headed south from Sydney with a small party and camped at the base of the basalt, making daily excursions to the top. While his convict crew cleared the summit of trees, Mitchell plotted and measured distant peaks and sketched the skyline.
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 (pyramid frame) supporting a visible disc above the ground mark.[2]
Many trigonometrical stations were placed on hilltops around Hong Kong. They strongly resemble those used in other former British colonial territories such as Australia, consisting of a white column topped with a black band.
In Japan, there are five classes of triangulation stations (三角点,sankakuten, lit. 'three corner points'):
As of August 2023[update] there are 5,765 trig stations inNew Zealand. They are placed on top of hills and are usually black and white.[5]
South Africa has a network of approximately 28,000 trig beacons, established by theChief Directorate: National Geo-spatial Information (historically known as the Trigonometrical Survey).[6] These beacons are typically white-painted concrete pillars supporting black metal plates in a cross shape, installed on mountains, hills or tall buildings.
In Spain there are 11,000 triangulation stations, concrete structures which typically consist of a cylinder120 cm high and30 cm in diameter over a concrete cubic base.
They were erected by theInstituto 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."
In the United Kingdom, trig points are typically concrete pillars and were erected by theOrdnance Survey.
The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurateretriangulation of Great Britain. TheOrdnance Survey's first trig point was erected on 18 April 1936 nearCold Ashby, 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 (nearLittle Ouse, Cambridgeshire, TL61718 89787).[7] 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.
In most of the UK, trig points are truncated square concrete (occasionally stone)pyramids orobelisks tapering towards the top. On the top a brass plate with three arms and a central depression is fixed, known as a "spider": it is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points. Abenchmark 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 on a plaque called a "flush plate". 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 toBrigadier Martin Hotine (1898–1968), head of theTrigonometrical andLevelling Division of the Ordnance Survey.[8] Many of them are now disappearing from the countryside as their function has largely been superseded byaerial photography anddigital mapping usinglasers andGPS. To quote from a page at the OS site: "Like an iceberg, there is more of trig pillar below the surface than above it."[9] 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."
The United StatesNational Geodetic Survey (NGS) and predecessor agencies manages theNational Spatial Reference System (NSRS), which includes permanent survey marks for horizontal position (latitude and longitude), height, or gravity. Some marks have information for both horizontal position and height. Some marks were established by NGS. Others were established by other organizations, such as state highway departments, but are included in the database that makes up the NSRS. Information about marks is available to the public online. The number of points in the NSRS is over 1,500,000.[10]
The image is the spire of theFair Haven, Vermont First Baptist Church, a horizontal survey mark (triangulation station), and is described in the NGS National Spatial Reference System under the permanent IDOD1373.