Trig point - Definition and Overview

Trig points are triangulation pillars erected by the Ordnance Survey in the UK.

The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurate re-mapping of Great Britain by triangulation directed by the cartographer and mathematician Martin Hotine. In low lying or flat areas some trig points may be only a few metres above sea-level. 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. 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 points are truncated square concrete pyramids tapering towards the top. On the top a brass plate with three arms and a central depression is fixed. A plaque on the side provides the reference number of the trig point and the letters O S B M ("Ordnance Survey Bench Mark"). Within the trig point, there are concealed mountings for a specialized theodolite, which was temporarily mounted on the trig point while measurements were taken.

Many trig points are now disappearing from the countryside as they no longer have a function since aerial photography and digital mapping using lasers and GPS co-ordinates has taken over.

Benchmarks have a similar purpose in providing a definitive height reference for mapping purposes.

See also

• Ordnance Survey
• Retriangulation of Great Britain

External links

• United Kingdom trig points (http://jeremyp.net/trigpoint/index.php)