... for the accurate mapping of Great Britain by the Ordnance Survey (OS).
**History:**
* **Early Surveys:** Prior to the 1930s, mapping in Britain relied on the "Principal Triangulation" which had been carried out between 1783 and 1853 and was a more piecemeal collection of observations.
* **The Retriangulation (1935-1962):** As Britain developed, there was a need for more detailed and accurate maps. Led by Brigadier Martin Hotine, the Ordnance Survey embarked on a new project to create a complete and consistent survey control network. The first trig pillar was installed in Cold Ashby, Northamptonshire, in 1936.
* **Design:** Most trig points are characteristic four-sided, tapering concrete pillars, about 1.2 meters (4 feet) high, designed by Brigadier Hotine to provide a stable base for the surveying instruments. Some variations exist, including stone-built ones and taller cylindrical ones in Scotland known as "Vanessas."
* **Obsolescence:** With the advent of modern technology, particularly GPS and satellite surveying techniques, trig points became functionally obsolete for their original mapping purpose by the 1990s. The OS now uses a network of GPS stations (OS Net) for its mapping operations.
**Legacy:**
Despite no longer being actively used for surveying, Ordnance Survey triangulation stations remain iconic landmarks in the British countryside. They are often found on hilltops and prominent locations, serving as navigational aids for hikers and a testament to the ingenuity and dedication of the surveyors who mapped the nation. Many people now engage in "trig bagging" or "trig pointing," a hobby of visiting as many of these historic markers as possible.