Happy 80th Birthday to the trig pillar

The first trig pillar appeared on the British landscape in 1936. It was the start of a network of over 6,500 that let OS map Great Britain to a high degree of accuracy.

On 18 April 1936 a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby to begin the retriangulation of Great Britain. That trig pillar is still standing 80 years on, along with thousands more around the country.

The trig pillar is quintessentially British, and even made it onto Bill Bryson’s list of favourite British items in his 2015 book The Road to Little Dribbling. But while many people know they’re something to do with OS, they’re often not sure quite how.

The Cold Ashby pillar was the start of a state-of-the-art triangulation network built to re-map Britain, dreamt up by Brigadier Martin Hotine. Some 6,500 were built and OS surveying teams spent 26 years gathering measurements across Britain to create a highly accurate map of the country.

Although more than 6,500 trig pillars were built, hundreds have been lost to housing developments, farming, coastal erosion and other causes. The greatest source of information on trig pillars (and other OS surveying marks) is www.trigpointing.uk. Users regularly ‘bag’ trig pillars and take photos to track their condition.

While there are many trig-baggers out there, trig-bagger extraordinaire Rob Woodall completed his 14-year mission to bag all of Britain’s trig pillars on their 80th anniversary. He’ll have bagged 6,190 trig pillars, a seriously impressive achievement. We joined his final bagging expedition at Benarty Hill in Fife to award him a mounted flush bracket to mark the moment.

We’re also asking people to get involved in the trig pillar birthday celebrations and have put together The Trig Pillar Trail Challenge to encourage people to #GetOutside and use OS Maps.

Rob Woodall, our #GetOutside champions and some of us at OS, have nominated our favourite trig pillars around the country. Many of our champions have also put together walking, running or cycling routes for people to reach the trig pillars using OS Maps.

You can win limited edition #TrigPillar80 T-shirts by sharing your trig pillar photos with us on Twitter or Instagram using #TrigPillar80. If you think few people would want to take a trig pillar photo, take a look on Instagram using #trigpoint – there are over 4,500 on there! We’re tagged in these photos every day – the top three themes at the moment seem to be the general view from hill tops, plonking the dog on top and doing yoga poses!

Top trig trivia

  1. Over 6,500 trig pillars were built for the retriangulation. We think about 6,000 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.
  2. Measuring angles by eye from a trig pillar meant the retriangulation relied on good weather – perhaps this is partly why it took until 1962 to complete! Modern GNSS surveying works in all weathers and is available 24 hours a day.
  3. Like an iceberg, there's a large part of the trig pillar below the surface.
  4. The highest trig pillar, unsurprisingly, sits atop Ben Nevis. The lowest trig pillar is at Little Ouse, one metre below sea level!
  5. Trig pillars are mostly made of cast concrete, but a few are built from local stone cemented together. The vast majority of trig pillars follow the standard Hotine design, but there are some ‘Vanessas’ which are taller, cylindrical concrete.

Find out more

The trig pillars that helped map Great Britain – BBC News in Pictures

OS Net – our modern, highly-accurate positioning network