What exactly is a trig point for? Don’t they just mark a summit?

Firstly, trig points aren’t found on all mountains; and even when they are, they might not be at the summit (we’ll come to that in a bit).

In 1935 Ordnance Survey began the ‘Retriangulation of Great Britain’ to provide a more accurate mapping of the country than that provided by the original triangulation (the process of determining the location of a point by forming triangles to it from known points) carried out between 1783 and 1853. OS built a network of triangulation stations – concrete pillars onto which super-accurate scientific angle-measuring survey devices, or theodolites, could be mounted. Thus ‘trig points’ began their colonisation of the nation.

For the system to work, from each trig point another two had to be visible in order to form the triangles from which to calculate positions. Because the network covered the length and breadth of Britain, many of the trigs had to be constructed in mountainous landscapes where, in order to ensure maximum visibility, they were usually built near the summits. But ‘near’ is the key word. They don’t always mark the highest point of the hill – just a position from which another two trig points or more can be viewed.

At least, that would have been the case. With lasers and satellites replacing trigs and theodolites, the concrete pillars are no longer required. Many are slowly deteriorating at the mercy of the elements, and others have disappeared altogether, while vegetation and modern constructions often block the line of sight between those that remain. They still have their uses, though. Pinpointed as they are on OS mapping, they’re an excellent navigation aid, and most hillwalkers will admit to being rather fond of them. Perhaps this is because, even if they don’t mark the actual summit, they’re usually a sign that you’re very nearly there.