Trig pillars – we salute you

They stand as shining white monoliths and there are 5,500 of them still standing in Great Britain.

And as the nation waves Union Flags aloft during the Jubilee celebrations, spare a thought for a few hardy souls, who are gathering to commemorate the last-used trig pillar in the retriangulation of Britain at Thorney Gale in Westmorland, Cumbria.

Monday, June 4 will mark half a century since this trig pillar was last used, bringing an end to the crucial role of trig pillars in the retriangulation of Great Britain.

Perhaps instantly recognisable to walkers, hikers and geography pupils, the quintessentially British trig pillars were once part of a state-of-the-art network built from 1936, to begin the retriangulation of Great Britain. The task was massive and lasted up until 1962. The benefits of this process we are still reaping today.

An Ordnance Survey team working on the retriangulation

Triangulation is basically a mathematical process that makes accurate map-making possible. It works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline and in this case, those known points were the 6,500 trig pillars erected across the country. In practice, a theodolite would have been secured to the top mounting plate and made level. It would then be directly over the brass bolt underneath the pillar. Angles were then measured from the pillar to other surrounding points. For the highest accuracy primary points in the retriangulation, many rounds of angles would have been measured with the observations taking several hours.

A new map-making control network for the whole country was badly needed at the time. In the early 20th century, map making was still based on a piecemeal collection of observations taken between 1783 and 1853. With the rapid development going on after the Great War accurate map making was a huge challenge.

Modern map-making has moved on. Yet our trig pillars still act as a beacon for weary ramblers and a symbol of what our great nation can achieve.

Top trig trivia

1. The survey control network of trig pillars was accurate to 20 metres over the entire length of Great Britain. Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the same area.

2. 6,500 trig pillars were built for the retriangulation of which around 5,500 are still standing. In total the retriangulation had in excess of 30,000 coordinated points. The modern OS Net network performs the same function with just 110 points.

3. Measuring angles by eye from a trig pillar meant the retriangulation was reliant on good weather – perhaps part of the reason it took until 1962 to complete! Modern GNSS surveying works in all weathers and is available 24 hours a day.

4. Trig pillars are mostly made of cast concrete but a few are built from local stone cemented together.

5. Like an iceberg, there is more of trig pillar below the surface than above it.

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