Shropshire History


Trig Points


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Trig Point on Brown Clee Hill (Paul Russon)


A trig point (properly known as a triangulation pillar) is a fixed surveying station that was used by the Ordnance Survey to accurately map the UK. From each trig point, it was possible to see and take a compass bearing on at least two other trig points, thus forming a triangle. To help in this, the top of a trig point has a metal plate that provides a mounting point for a theodolite.


Many were located on hilltops for the purposes of visibility, but subsequent vegetation and tree growth means that some no longer have a line of sight. In low lying areas, there are no hills to site them and the lowest is 1 metre below sea level at Little Ouse, Norfolk.


A big project by the Ordnance Survey began in 1935 with the aim of accurately re-mapping the whole of the UK. Brigadier Martin Hotine, head of the Trigonometrical and Levelling Division, invented the concrete trig point as we know it and the first one was erected on 18th April 1936 near Cold Ashby in Northamptonshire.



Building and using trig point at Cold Ashby (Ordnance Survey)


Cold Ashby trig point today (Ordnance Survey)


When all the trig points were in place, it was possible to measure the bearings between each trig point. This allowed the construction of a series of triangles that were the basis of mapping the UK. At one time, there were around 6,500 trig points in the UK but they are no longer used, their function being replaced by aerial photography and digital mapping. Although the Ordnance Survey no longer uses the trig points, it is still responsible for maintaining them and there are around 6,000 left. Nowadays they are more used as navigational aids for hill walkers.





A detailed list of trig points in Shropshire can be found on the excellent website  Trigpointing UK