And so it was that a day after the Peak District National Park celebrated its 75th birthday, another icon of the British landscape climbed out of bed, put on its pipe and slippers and said ‘move aside youth’.
Happy 90th birthday today to the humble ‘trig point’.
The Ordnance Survey’s first trig point was erected on 18 April 1936 near Cold Ashby, Northamptonshire.
The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurate retriangulation of Great Britain.
When all the trig points were in place, it was possible in clear weather to see at least two other trig points from any one trig point.
Careful measurements of the angles between the lines-of-sight of the other trig points then allowed the construction of a system of triangles which could then be referenced back to a single baseline to construct a highly accurate measurement system that covered the entire country.
In most of the UK, trig points are truncated square concrete (occasionally stone) pyramids or obelisks tapering towards the top.
On the top a brass plate with three arms and a central depression is fixed, known as a “spider”: it is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points. A benchmark is usually set on the side, marked with the letters “O S B M” (Ordnance Survey Bench Mark) and the reference number of the trig point on a plaque called a “flush plate”.
Within and below the visible trig point, there are concealed reference marks whose National Grid References are precisely known.
The standard trig point design is credited to Brigadier Martin Hotine (1898–1968), head of the Trigonometrical and Levelling Division of the Ordnance Survey. Many of them are now disappearing from the countryside as their function has largely been superseded by digital mapping techniques and GPS.
Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the entire length of Great Britain.
There are over 6500 trig points still visible in the UK.
Ben Nevis has the highest trig pillar; the lowest is Little Ouse, at -0.533m below sea level, in Cambridgeshire.
How many times as hillwalkers, fell runners and mountaineers have you and your lungs been grateful to finally reach one of these bloody things on a day out in the British mountains ?
Happy birthday Trig 🥳🥳🥳🥳