Trigpoints are the common name for “triangulation pillars” the UK mapping and triangulation system before GPS and Google Earth. There is a great Trigpoint website with map references pictures and search facilities. ‘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 compass built into a telescope) on the top of the pillar, accurate bearings to nearby trigpoints could be taken. 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”, and it is essential that you are familiar with this system if you are to get the most of OS maps, or this website. ‘