Triangulation station

Trigonometrical stations form networks of triangulation. 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 the Global 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 even aviators as navigational aids. Particular small triangles on maps (◬) mark summits, for example.

The nomenclature for triangulation stations 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,{{cite web|title=Surveying for Mapping — Section 1, Introduction|url=http://www.icsm.gov.au/mapping/surveying1.html#jargon_trig|work=Intergovernmental Committee on Surveying and Mapping|publisher=Commonwealth of Australia|access-date=7 November 2012|archive-date=3 November 2012|archive-url=https://web.archive.org/web/20121103061049/http://www.icsm.gov.au/mapping/surveying1.html#jargon_trig|url-status=dead}} and trig 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.{{Cite web |title=Frequently Asked Questions Maps And Mapping |url=https://www.usgs.gov/faqs/how-can-i-find-survey-benchmark-and-triangulation-station-information |website=USGS.gov}}

Jellore_trig_plaque.jpg|Australia's first trig

Trigonometrical Station on Mt Gibraltar, NSW, Australia.jpg|Trigonometrical station, NSW, Australia

Trig point atop Mount Wellington, Tasmania.jpg|Trig point atop Mount Wellington, Tasmania

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.

Chek Lap Kok Scenic Hill trigonometrical station.jpg|A trigonometrical station in Chek Lap Kok Scenic Hill, Hong Kong

In Japan, there are five classes of {{nihongo|triangulation stations|三角点|sankakuten|lit. 'three corner points'}}:

;{{nihongo|Class 1|一等三角点|ittō sankakuten}}

:They are installed approximately every {{convert|40|km|mi}}, with smaller ones (as necessary) about every {{convert|25|km|mi}}.{{cite web|url=http://www.gsi.go.jp/MUSEUM/TOKUBE/KIKA5-sanka1.htm|title=一等三角測量とは|language=ja|trans-title=About Class 1 Triangulation Stone Markers|access-date=20 April 2015|publisher=Geospatial Information Authority of Japan|archive-date=27 April 2015|archive-url=https://web.archive.org/web/20150427143508/http://www.gsi.go.jp/MUSEUM/TOKUBE/KIKA5-sanka1.htm|url-status=dead}} There are about 1000 throughout Japan. The pillars are {{convert|18|cm|in}} on a side, and each pillar is anchored with two very large perpendicular rocks buried underground.{{cite web|url=http://www.gsi.go.jp/MUSEUM/TOKUBE/KIKA5-sanka6.htm|title=一等三角点標石の構造|language=ja|trans-title=Class 1 Triangulation Stone Marker Construction|access-date=20 April 2015|publisher=Geospatial Information Authority of Japan|archive-date=27 April 2015|archive-url=https://web.archive.org/web/20150427143506/http://www.gsi.go.jp/MUSEUM/TOKUBE/KIKA5-sanka6.htm|url-status=dead}}

;{{nihongo|Class 2|二等三角点|nitō sankakuten}}

:They are installed approximately every {{convert|8|km|mi}}. There are about 5000 throughout Japan, and the pillars are {{convert|15|cm|in}} on a side. Each pillar is anchored with a very large perpendicular rock buried underground.

;{{nihongo|Class 3|三等三角点|santō sankakuten}}

:There are about 32,000 installed throughout Japan, with one approximately every {{convert|4|km|mi}}. The pillars are {{convert|15|cm|in}} on a side, and each pillar is anchored with a large perpendicular rock buried underground.

;{{nihongo|Class 4|四等三角点|yontō sankakuten}}

:They are installed approximately every {{convert|2|km|mi}}, and there are about 69,000 throughout Japan. The pillars are {{convert|12|cm|in}} on a side, and each pillar is anchored with a large perpendicular rock buried underground.

;{{nihongo|Class 5|五等三角点|gotō sankakuten}}

: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.

3rd class triangulation point in Shiroyama Park in Inagi.jpg|Class 3 triangulation point in Shiroyama Park in Inagi, Tokyo

{{See also|Cartography of New Zealand}}

{{As of|2023|August}} there are 5,765 trig stations in New Zealand. They are placed on top of hills and are usually black and white.{{Cite web |title=NZ Trig Points (Topo, 1:50k) |url=https://data.linz.govt.nz/layer/103476-nz-trig-points-topo-150k/ |access-date=23 September 2023 |website=data.linz.govt.nz}}

Mount Maunganui triangulation station.jpg|Trig station on top of Mount Maunganui

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).{{cite web |url=http://www.ngi.gov.za/index.php/what-we-do/geodetic-and-control-survey-services/36-trignometrical-beacons-town-survey-marks-bench-marks |title=Trigonometrical beacons, town survey marks & bench marks |publisher=Chief Directorate: National Geo-spatial Information |access-date=14 January 2018}} These beacons are typically white-painted concrete pillars supporting black metal plates in a cross shape, installed on mountains, hills or tall buildings.

P1000268a The summit of Lion's Head.jpg|Trig beacon on the summit of Lion's Head in Cape Town

In Spain there are 11,000 triangulation stations, concrete structures which typically consist of a cylinder {{nowrap|120 cm}} high and {{nowrap|30 cm}} in diameter over a concrete cubic base.

They were erected by the Instituto Geográfico Nacional, usually painted in white, and may be marked with a metallic label with the warning: "The destruction of this sign is punishable by law."

Cima de Peñalara. Parque nacional de la Sierra de Guadarrama. España, Spain.jpg|Trigonometric station on the top of Peñalara mountain (Spain)

In the United Kingdom, trig points are typically concrete pillars and were erected by the Ordnance Survey.

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. The Ordnance Survey's first trig point was erected on 18 April 1936 near Cold 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 (near Little Ouse, Cambridgeshire, TL61718 89787).[https://www.ordnancesurvey.co.uk/blog/2016/04/happy-80th-birthday-to-the-trig-pillar/ "Happy 80th birthday to the trig pillar" Ordnance Survey] 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 or obelisks 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. A benchmark 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 to Brigadier Martin Hotine (1898–1968), head of the Trigonometrical and Levelling Division of the Ordnance Survey.{{cite web|url=https://www.telegraph.co.uk/travel/731568/Britain--Master-of-all-he-surveys.html|archive-url=https://web.archive.org/web/20080627114223/http://www.telegraph.co.uk/travel/731568/Britain--Master-of-all-he-surveys.html|url-status=dead|archive-date=27 June 2008|title=Britain: Master of all he surveys|last=Crane|first=Nicholas|date=30 October 2004|work=The Daily Telegraph|access-date=24 January 2010|location=London, England}} Many of them are now disappearing from the countryside as their function has largely been superseded by aerial photography and digital mapping using lasers and GPS. To quote from a page at the OS site: "Like an iceberg, there is more of trig pillar below the surface than above it."[http://blog.ordnancesurvey.co.uk/2012/05/trig-pillars-we-salute-you/ 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."

Trig Point near Wootton Wawen.jpg|A trig point near Wootton Wawen, Warwickshire, England

Trig point.jpg|Sign at Triangulation Point 3M of the Israeli Cassini Soldner measuring system outside Ofakim.

The United States National Geodetic Survey (NGS) and predecessor agencies manages the National 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.{{Cite web

| title = What is the National Spatial Reference System?

| work = National Geodetic Survey

| access-date = 1 December 2023

| date = 28 October 2019

| url = https://geodesy.noaa.gov/INFO/facts/nsrs.shtml}}

The image is the spire of the Fair Haven, Vermont First Baptist Church, a horizontal survey mark (triangulation station), and is described in the NGS National Spatial Reference System under the permanent ID [https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=OD1373 OD1373].

OD1373-horizon.jpg|Fair Haven First Baptist Church

• Struve Geodetic Arc
• Benchmark (surveying)
• Bilby tower
• Boundary marker
• Geodesy
• Ordnance Survey
• Retriangulation of Great Britain
• Survey marker
• Trigpointing is a pastime in which people individually go out, find and log the location of trig points.
• Cover of Martyn Bennett's album Grit

{{Reflist}}

{{Commons category|Trig points}}

• [http://www.trigpointinguk.com/ TrigpointingUK]
• [http://www.haroldstreet.org.uk/waypoints/ GPS Waypoints of all UK Trigpoints]
• [https://www.geograph.org.uk/search.php?i=5157221 Photos of examples of trig points in the UK on geograph.org.uk]
• [http://jeremyp.net/trigpoint/index.php Database of trig points in the United Kingdom]
• [https://geodesie.ign.fr/index.php?page=reperes_de_nivellement/ Repères de nivellement. French I.G.N]
• [https://geodesie.ign.fr/fiches/index.php?module=e&action=visugeod/ Serveur de fiches (maps). French I.G.N]
• [http://apps.linz.govt.nz/gdb/ Database of trig points in New Zealand] {{Webarchive|url=https://web.archive.org/web/20151216123623/http://apps.linz.govt.nz/gdb/ |date=16 December 2015 }}
• [http://blog.ordnancesurvey.co.uk/2012/05/trig-pillars-we-salute-you/ "Trig pillars we salute you", blog post from UK Ordnance Survey]

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Category:Surveying and geodesy markers