History

Trig points are the common name for triangulation points or pillars. 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 trig points could be measured. A major project to map out the shape of Great Britain began in 1936. The network of triangulation points, 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.

Types of trigs

Pillar

The classic trig point. A concrete pillar about 4 foot high, with a flush bracket near the base and a circular plate on top with groves for mounting a theodolite

Intersected station

These are 'non-occupiable' stations consisting of church spires, chimneys or masts.

Bolt

A bolt is a small survey marker, always placed in natural rock