Clach MacKenny

A trigonometrical station / point, triangulation pillar, trig beacon or trig point is a fixed surveying station for the geodetic surveying and other surveying projects on nearby areas. They are generally known as trig points in the United Kingdom,. The station is usually set up by a government with known coordinate and elevation published. Many stations are located on the peaks of hills for the ease of being spotted from many directions. A graved metal plate is on the top of a pillar for securing a theodolite or reflector. Trigonometrical stations are grouped together to form a network of triangulation. Positions of all land boundaries, roads, railways, bridges and many other infrastructures can be accurately located by the network.

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. 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.

In most of the United Kingdom, 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. A benchmark is set on the side, marked with the letters "O S B M" (Ordnance Survey Bench Mark) and the reference number of the trig point. Within the trig point, there are concealed mountings for a specialised theodolite, which was temporarily mounted on the trig point while measurements were taken. Many of them are now disappearing from the countryside as their function has largely been superseded by aerial photography and digital mapping using lasers and GPS measurements.

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