Trig point N53 07.329 W3 10.518

Trig points 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Their use has now been superseded by aerial photography and satellite mapping, and some have been removed so as to restore the natural state of the landscape they stood on.