27 March 2008 19:10
What is a trig point and what is it for?
By Country Walking magazine
Look in the pages of Country Walking and Trail and you might think they were for sitting on for pictures.
Rather more sensibly, they are the remains of a massive Ordnance Survey project to map Great Britain with absolute accuracy from 1935 onwards.
The trig point – or triangulation point - was instrumental in this geodetic survey. They were workstations and reference points for the surveyor, who could attach his theodolite equipment to the fixtures and fittings within the column, including the three-pronged metal plate in the top of the trig point.
The location of each trig point was selected so that at least two others would be visible from it. Using these, the surveyor could work out the angles on the lines of sight between the three points and create a triangular mapping grid – hence, triangulation.
Each trig point also contains another plate, usually low down on one side, featuring the bench mark of that particular trig point and the letters OSBM, for Ordnance Survey Bench Mark.
Their use has now been superseded by aerial photography and satellite mapping, and some have been removed so as to restore the natural state of the landscape they stood on. Most remain, however, as they’re a massively useful navigational aid for walkers, not least for the simple act of confirming you’ve reached the summit of something, especially in mist.
In some cases, they’ve been embraced as part of the heritage of the hills. On Brown Clee in Shropshire, for instance, the trig point was removed to make way for a snazzy new viewfinder, but the theodolite plate and bench mark were saved and inserted into a new flight of stone steps leading to the summit.
Oh – and trig points don’t have to be high, either. Many in places such as Lincolnshire and Cheshire are barely above sea level, but the logic was the same – provided they yielded a viewpoint from which two others could be seen, they did the job.
So the humble trig point helped to draw the maps we survive on. And with those views, they make a peachy spot for a picnic too.