What is a Trig Pillar Anyway?

So What Are Trig Pillars?

Started by the Ordnance Survey in 1936, triangulation pillars were created by the surveyors to provide bases for the mapping equipment of the day, the humble theodolite. This was an impressive feat given the times, and helicopter dropping of equipment was not really an option.  Teams had to drag all of the equipment to make the pillar and perform the measurements to the top of the highest points in Britain  – and the somewhat easier lowest pillar TP4449 in Little Ouse near Ely, Norfolk at 0.5 m below sea level.  

The pillars themselves were used to mount a theodolite and take the necessary readings from a solid base. Teams would then utilise some nasty maths, the kind that would induce nightmares in many of us, to map distances and elevation, building a network of the entire country. They can be seen on OS maps as a blue triangle with a dot in the centre.

Types of Trig Pillar

The traditional square based concrete pillars we all know and love are sometimes referred to as Hotine Pillars in reference to the Brigadier Martin Hotine, who was responsible for the Trigonometric and Levelling Division at OS, at the time of inception. 

Though most pillars resemble these Hotine pillars there are also “Vanessas” which are taller with a slender and more shapely figure.  Others are made of local stones concreted together. Most pillars will have a “flush bracket” which will denote their height above sea level.  

Trig Pillar - The Cloud TP6366

The Ordnance Survey have a great blog on the history of the humble Trig Pillar, including articles on the Theodolites used to do the triangulation and even details on how Ben Nevis came to grow by a metre

Trig Bagging

Trig bagging itself is nothing new. In fact, one Rob Woodall has even managed to bag all 6190 UK pillars. Let’s hope one day we can get close, though it took him 14 years so we have a way to go!


So why do we want to bag them? Well, why not?  It gets us outdoors, lets us explore new places, and hopefully meet some new people. Join us on our journey on this blog and follow us on Instagram for regular updates!

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