What is a Trig Point?

A triangulation station, also known as a trigonometrical point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity. They are no longer in use, however, Ordnance Survey still own a large number of them across the UK.

As well as the familiar pillars, the Ordnance Survey also installed a large number of smaller survey marks. I've logged some of these here.