Triangulation pillars, trig pillars or trig points were erected by the Ordnance Survey (OS) when it began the retriangulation of Great Britain. Surveyors erected the first white concrete pillar in a field in Cold Ashby in April 1936. Before then map making was based on the Principal Triangulation, piecemeal observations made between 1783 and 1853, but as the country developed rapidly in the 20th century, the system could not support the changes that accurate mapping required.
Brigadier Martin Hotine, head of the Trigonometrical and Levelling Division at OS, planned the retriangulation in 1935. It created the OSGB36 datum and the National Grid, which are still in use creating a network covering the whole country that unified mapping from county projections into a national system. He designed the trig pillar as a base for the theodolites used by survey teams, who located sites and carried the materials to build them.
Triangulation works by measuring angles to known points at either end of a fixed baseline to determine the location of a point. The known points were more than 6,500 trig pillars that were erected. A theodolite was fastened to the mounting plate on top of the pillar and levelled so that it was directly over a brass bolt. Angles were measured from the pillar to surrounding trig points. Trig pillars have since been superseded by technology.
Many trig pillars are on hill peaks, but the lowest, at Little Ouse in Norfolk, is three feet (0.9 m) below sea level. Many trig pillars have been lost, but most survivors are the standard Hotine design. A few are made of stone, and in Scotland are some taller, cylindrical concrete pillars or Vanessas. The Ordnance Survey is responsible for maintaining the pillars.
Most trig pillars are no longer used, but around a thousand have been refurbished with bolts and rivets and accurately measured using GPS (Global Positioning System) technology to form the “Passive Station” network. Thirty “active stations” also transmit positional information which can be received by survey-quality GPS receivers.