A history of the trig pillar


The trig pillar network

Yesterday marked 80 years since the trig pillar was first used in the retriangulation of Great Britain on 18 April 1936. On that day, a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby and began the retriangulation of Great Britain.

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. But what is the background to the trig pillar?


What was the retriangulation of Great Britain?

Triangulation is a mathematical process that makes accurate map making possible. In the early 20th century, map making was still based on the Principal Triangulation which was a piecemeal collection of observations taken between 1783 and 1853. The system was starting to collapse and couldn’t support the more accurate mapping needed to track the rapid development of Britain going on after the Great War.

A diagram we found in the OS archives of the mountains from which readings were taken

A diagram we found in the OS archives of the mountains from which readings were taken when surveying Ben Nevis

In 1935 Ordnance Survey, in a project led by Brigadier Martin Hotine, decided to implement a complete new control network for the whole country and at the same time unify the mapping from local county projections onto a single national datum, projection and reference system. This lead to the OSGB36 datum and The National Grid, both of which are still with us today.

Who designed the trig pillar and how did it help with the retriangulation?

The man responsible for the trig pillar that we all recognise today was Brigadier Martin Hotine. Born in 1898 in Wandsworth, London, Hotine became head of the Trigonometrical and Levelling Division at OS. The Brigadier was responsible for the design, planning and implementation of the retriangulation and he designed the iconic trig pillar. He designed them to provide a solid base for the theodolites used by the survey teams to improve the accuracy of the readings obtained. As a result, they are sometimes referred to as ‘Hotine Pillars’.

Hotine, second right, planning the retriangulation

Hotine, second right, planning the retriangulation

If you haven’t come across it before, triangulation 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. But time and technologies have moved on enormously to the point where the traditional trig pillar is now obsolete in its original guise. We’ll be talking about how we map Britain in 2016 on the blog tomorrow.

The inner workings of a trig pillar

You can hear more about surveying using trig pillars in this video.

A testament to surveyors of the past

Although 6,500+ trig pillars were built, hundreds have been lost to housing developments, farming, coastal erosion and other causes. The vast majority follow the standard Hotine design, but some are stone built, and in Scotland there are some ‘Vanessas’ which are taller, cylindrical concrete pillars.

A 'Vanessa' trig pillar on the Isle of Skye, photographed by Scott MacLucas-Paton

A ‘Vanessa’ trig pillar on the Isle of Skye, photographed by Scott MacLucas-Paton

You can only imagine how hard it was for surveyors of the past to not only map Britain, but to also locate sites for trig pillars and carry the materials to remote sites to then build the trig pillars too.

Setting a trig pillar

Setting a trig pillar

Carrying materials up Sca Fell

Carrying materials up Sca Fell

It’s a true testament to their skills that such an accurate map of Britain was created from such humble beginnings as the trig pillar 80 years ago.

Find out more and get involved in the #TrigPillar80 celebration

Read yesterday’s blog about the 80th birthday celebrations, and join us tomorrow to find out how we survey and map Britain today.

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

Take part in The Trig Pillar Trail Challenge and share your trig pillar photos with us using #TrigPillar80 on Twitter and Instagram – and you could win a limited edition T-shirt.

You may also like

Britain’s top trig-bagger adds FBMs to his haul
Benchmark or trig pillar: what’s in a name?
Taking White Hart Lane off the map
Queensferry Crossing is firmly on the map

10 Responses

  1. Hello,

    I’m writing from a production company called betty. We are going to filming a segment about trig pillars and would be very grateful if someone could get in touch with us in regards to theodolites.

    my number here is 0207 290 0206 thank you!

    1. Hi Elena

      I believe my colleague Andy has dropped you an email in the last half hour or so, so you should be able to move this one forward.

      Thanks, Gemma

  2. Roger Roberts

    Is it true that the first trig point is at Old Sarum Salisbury. I can remember seeing a monument near there or on the road from Amesbury to Salisbury.

    1. Hi Roger

      Our first trig pillar was over in Cold Ashby, Northamptonshire and has a plaque on there. It was first used in the retriangulation of Great Britain on 18 April 1936 – so that’s why we’ve been celebrating the 80th anniversary of the trig pillar this year. Hope that helps.

      Thanks, Gemma

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  4. Ann H.

    Hi Gemma,

    I have been trying to find out if there are any books on the OS and trig pillars. Would you happen to know if there are? And if not I think it would make a great guide book for those trig-baggers out there if anyone feels disposed to write one.

  5. How was the trig pillar used in measurements? I understand the principle of triangulation but the datum point was presumably at ground level whereas the theodolite was a finite (and constant) height above this. Was there a simple constant used in obtaining ground heights?

    Thank you for this opportunity to ask – Keith

    1. Looking at the cross sectional diagram of a pillar will help with this explanation from our Geodetics team.

      The lower bolt was so the point could easily recreated if the pillar and top bolt were destroyed. The top bolt (roughly ground level in centre of pillar) was the mark that was being coordinated when the pillar was observed from or to. During the construction of the pillar the spider (three legged brass “device”) which holds a theodolite on the top of the pillar was centred over the top bolt so a theodolite placed on the spider was automatically centred over the bolt.

      Nearly every pillar has a flush bracket on the side which is the (above mean sea level) height reference mark for the pillar. Due to variations in pillar construction a constant height from FB to pillar top cannot be guaranteed so the height of an instrument during a survey would always be measured from the FB. This ties the survey observations into height above mean sea level.

      Many thanks

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