Trigpoints are the common name for "triangulation pillars". These are concrete pillars, about 4' tall, which were used by the Ordnance Survey in order to determine the exact shape of the country. They are generally located on the highest bit of ground in the area, so that there is a direct line of sight from one to the next. By sitting a theodolite (an accurate protractor built into a telescope) on the top of the pillar, accurate angles between pairs of nearby trigpoints could be measured. This process is called "triangulation".
A major project to map out the shape of Great Britain began in 1936. The network of triangulation pillars, with accurately known positions, led to the excellent OS maps which we enjoy today. The coordinate system used on these maps is known as the "National Grid"
6,500 Trig Pillars were built for the Retriangulation of which around 5,500 are still standing. In total the Retriangulation had in excess of 30,000 coordinated points. The modern OS Net network performs the same function with just 110 points.
The highest Trig pillar in the UK stands at 1,345 m at the summit of Ben Nevis
The lowest can be found here in the flat fens at -1 m.
Terrain rating reflects that a riverbank needs to be negotiated. Please expect long grass and nettles during the summer months.