Trig point
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A Trig point is the common name given to trigonometrical stations in the United Kingdom and Australia. In the UK they 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 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.
In most of the United Kingdom, trig points are truncated square concrete (occasionally stone) pyramids or obelisks tapering towards the top, generally known as Hotine Pillars; in parts of Scotland round 'Venessa' pillars are more common, requiring less concrete to build. 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 specialized theodolite, which was temporarily mounted on the trig point while measurements were taken. Many of these trig points are now disappearing from the countryside as their function has largely been superseded by aerial photography and digital mapping using lasers and GPS measurements.
A national geodetic survey and adjustment carried out in the early 1970s in Australia has left a legacy of trig stations consisting of a ground mark with a white quadripod supporting a black disc above the ground mark. These trig stations are clearly visible for many kilometres and useful for hikers.
See also
- Geodesy
- Ordnance Survey
- Retriangulation of Great Britain
- Trigpointing is a 'sport' which has grown for people to individually go out, find and log the location of trig points.
External links
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