Happy 80th birthday to the trig pillar

On 18 April 1936 a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby and began the retriangulation of Great Britain. That trig pillar is still standing 80 years on, along with thousands more around the country. We’re celebrating by sharing the story of the humble trig pillar, still much loved by walkers today, and giving you the chance to join our celebrations with The Trig Pillar Trail Challenge.

Where it all began - Cold Ashby

Cold Ashby photo by Bridgeman via Trigpointing UK, a great site for all things trig

What is a trig pillar?

Hotine, second right, planning the retriangulation

Hotine, second right, planning the retriangulation

The shining (sometimes) white monoliths are now instantly recognised by any walker, or geography lover and have inspired many a trigbagger. They’re 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 what were they for? Now largely redundant, back in 1936, they formed a state-of-the-art network built to re-map Britain, dreamt up by Brigadier Martin Hotine. Responsible for the design, planning and implementation of the retriangulation, Hotine also designed the iconic trig pillar to provide a solid base for the theodolites used by the survey teams to improve the accuracy of their readings.

Some 6,500 were built, to be used for triangulation, the 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, across the country. OS surveying teams spent 26 years gathering measurements across Britain to create a highly accurate map of the country, but time and technologies have moved on enormously to the point where the traditional trig pillar is now obsolete in its original guise. They still act as a beacon for many an outdoors lover, but they no longer help shape our maps. Look out for tomorrow’s blog where we’ll share more detail on the history of the trig pillar, how they were built and used, the inner workings and much more.

The trig pillar today

IMG_0903 1Although 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 Ordnance Survey surveying marks) is www.trigpointing.uk. Users on there 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 13-year mission to bag all of Britain’s trig pillars last weekend in Fife. He’s bagged 6,190 trig pillars, a seriously impressive achievement. We joined his final bagging expedition and awarded him a mounted flush bracket to mark the moment. Look out for Thursday’s blog with more details on his final trig-bagging adventure.

The Trig Pillar Trail Challenge

#TRIGPILLAR80We’d love you to get involved in the trig pillar birthday celebrations too and have put together The Trig Pillar Trail Challenge. 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 you to reach the trig pillars using OS Maps.

To celebrate the trig pillar and getting outside to explore Britain, if you take a photo of one, or with one, anywhere around the country, send us a photo on Twitter or Instagram using #TrigPillar80, and you could win a limited edition T-shirt.

How do OS survey today?

The modern equivalent is the OS Net network of 110 Global Navigation Satellite System(GNSS) receivers. Our surveyors use OS Net and GNSS technology everyday to instantly position new map detail to within a few centimetres. What took many hours at Cold Ashby in 1936 we can now do in seconds and to a far greater degree of accuracy. Find out more about our current surveying process later this week on the blog.

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 3mm over the same area.
  2. Over 6,500 trig pillars were built for the retriangulation of which we think somewhere in the region of 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.
  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 a large part of the trig pillar below the surface.
  6. There are many dedicated trigbaggers out there and even a website called trigpointing.co.uk. Rob Woodall will have bagged all the trigs still standing on 17 April 2016, a task which has taken 14 years to complete.
  7. While we no longer use the trig pillars, they are still our responsibility to maintain. If you do spot a trig pillar looking unsafe, let us know, so that we can take a look and decide on the best way to remedy it.
  8. Some trig pillars have been redecorated. We don’t condone this, but it’s preferable to those sorry ones that are covered in graffiti. We’ve seen an alien, a minion, Welsh dragons and English roses so far. Again, spot one in a particularly bad way and let us know so that we can see if we can sort it out.
  9. The highest trig pillar, unsurprisingly, sits atop Ben Nevis. The lowest trig pillar is at Little Ouse, sited at -1m!
  10. The vast majority of trig pillars follow the standard Hotine design, but there are some ‘Vanessas’ which are taller, cylindrical concrete pillars.

Don’t forget, you can get involved in the #TrigPillar80 celebrations by taking a walk to one of the 25 trigs in our Trig Pillar Trail Challenge, or by sharing your trig photos with us on Twitter and Instagram.

Take a look at our Flickr album with trig pillar photos past and present.

See the BBC gallery celebrating the 80th birthday of the trig pillar.

You may also like

7 fantastic things about #TrigPillar80
#TrigPillar80 competition now closed
#GetOutside with some trig-bagging this summer
#TrigPillar80 winners picked

7 Responses

  1. Paul Martin

    Hi Gemma,
    A great blog with very interesting information about Trig Pillars.
    I’ve been slowly locating Trig Pillars within Shropshire since 2004.
    I find the experience an added objective to my planned walks (the first one is not to get lost) using OS maps, which has taken me and my dear friend to many beautiful locations that would not otherwise have been discovered.
    In 2004 there were 106 Trig Pillars in Shropshire, but alas there are now only 99 remaining, I’m assuming the missing 7 have been removed by private land owners. I was also shocked to discover that the one I visited on top of the Brown Clee (Abdon Burf) has since been removed and have never understood why as this area is a well-known land mark.
    We now have just 8 left to discover and hope to accomplish this during the year of the Trig Pillar’s 80th Birthday. Some have been difficult to locate and have necessitated extensive use of secateurs to reveal. Needless to say I have photographed all of those visited with my mate and I in various poses.
    We even applied to adopt one of them in accordance with your organisation’s campaign, but unfortunately we were informed that we were too late and that the scheme had expired.
    Congratulations to Rob Woodall in locating 6,190 Trig Pillars during the past 14 years. What an amazing achievement!
    I look forward to seeing more information posted on your blog about these friendly monoliths.
    Please keep up the good work.
    Kindest regards,

  2. Roger Whittle

    I thin it was Nicholas Crane who told the story of Major Hotine and his near thirty year, world war spanning survey. I have since had a towering respect for his (and their) achievement, ranking alongside Whittle’s jet engine, Parson’s turbines, Brunel’s beidges and Concorde.

    I tell my Scouts what a Trig Point (Pillar) is, why it was done and how accurate it was when they finally completed it. To be able to hold my hands 17″ apart and tell them that after all the trudging up mountains, making a million night time measurements and suffering a world war, the difference was ‘this much’, actually brings a tear to my eye.

    Hotine and the OS deserve more recognition, for the finest maps in the world, bar none.

  3. Keir Polyblank

    I was put into the Royal Artillery to do my National Service in 1952, aged 18 (born, it seems, a couple of years before the first pillars were cast) With a mathematical bent I began training to be a surveyor,and saw my first trig point pillar on Salisbury plane, and learnt how to use a Watts Microptic theodolite. At the time I had no idea that there were so many pillars about the countryside. How conveniently the theodolte sat in the little grooves in the bronze plate, the whole system another first-class British invention, I might have thought.
    A few months later I was on the Korean peninsular, in a hostile battery locating unit, amongst other tasks, having to survey-in partially buried microphones that located the report from a hostile gun. When mics were blown out of the ground, or lost for other reasons, we’d have to go back to known points to survey-in new mics. Incredibly the known points, miles back from the front, were concrete pillars with a bronze plate set in the top identical to ours at home. It seems the Japanese, who had colonised Korea for many years, had triangulated the whole peninsular and set up these trig points, obviously copying the British system. Or was it the other way round? Does anyone have any knowledge of this?
    Out of range of a trig point, one pair of surveyors and their jeep driver would have to set up on any visible hill, and to help another pair sighting the first, they would erect a small triangular orange tent above the theodolite. Getting up on the hill top, taking readings and getting off again had to be done as quickly as possible; close to the frontline the orange tent was visible to Chinese spotters, and shells would soon begin to arrive!

  4. Sophie Dykes

    Hi Gemma
    Is there any way of finding out which surveying teams built which trig points? My dad was an OS surveyor in the 60s and his team built some of the trig points in the far north and west of Scotland. He died nearly 10 years ago, and I would like to visit one or two of “his” trig pillars in his memory, as he often told us tales of the places he’d stayed and the digs they stayed in, though I can’t remember any place names, just the anecdotes. He kept in touch with some of his team, but I think all of the guys we had contact details for have also passed away. Any advice would be gratefully received.

    1. Hi Sophie

      That sounds like a lovely idea to commemorate your dad. Unfortunately, there was no formal recording of who built each trig pillar that has been retained in the paper trig files from the time. As you can imagine, it was long before the time of digital records and the system our surveyors work on now that logs who carries out each job. Our employee records may contain details of job roles or general locations a surveyor was based. You could try contacting customerservices@os.uk with details of your father and our HR team may be able to glean some further information for you.

      Many thanks

  5. Mark Adamczyk

    I’m the proud owner of one of 12 Cooke, Troughton and Sims ‘Tavistock’ theodolites, that were purchased by the OS prior to the start of the re-triangulation of Great Britain in 1936 (according to the serial numbers of the instruments which are listed in the official history of the re-triangulation). I’m interested to know if any of the other instruments have survived, or if I can send you any pictures to put on your blog or Flickr page? Thanks. Mark (Halifax, Yorkshire)

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