Trig points 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 thecountry. 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 he next one. By sitting a theodolite 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 the shape of Britain began in 1936. the network of triangulation pillars, with accurately known positions, led to the excellent OS maps which we use today. Each pillar had its own unique number on the front called a flush plate. This trig is S3015.
FTF for Mum of George. congrats