Triangulation station

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

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.

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/a-history-of-the-trig-pillar "Happy 80th birthday to the trig pillar" Ordnance Survey]{{cite web

|author =

|date = 18 April 2016

|title = Happy 80th birthday to the trig pillar

|url = https://www.ordnancesurvey.co.uk/blog/happy-80th-birthday-to-the-trig-pillar

|website =

|location = Southampton, United Kingdom

|publisher = [[Ordnance Survey]]

|url-status =

|archive-url =

|archive-date =

|access-date = 19 Apr 2026

}} 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) [[pyramid]]s or [[obelisk]]s 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 (surveying)|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 [[Martin Hotine|Brigadier Martin Hotine]] (1898–1968), head of the [[Trigonometric]]al 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 [[laser]]s and [[Global Positioning System|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."

In most of the UK, trig points are truncated square concrete (occasionally stone) [[pyramid]]s or [[obelisk]]s 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 (surveying)|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 [[Martin Hotine|Brigadier Martin Hotine]] (1898–1968), head of the [[Trigonometric]]al 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 [[laser]]s and [[Global Positioning System|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."