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.

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A glimpse into the role of an OS surveyor
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Meet the team: Mark Cuthbert

68 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

    2. Fred Rees

      Hi Roger Roberts
      The ordnance mark you saw at Old Sarum Salisbury is in fact ‘gun end of base’ (still in situ, saw it this morning) and the other end was by Beacon Hill at Bulford.

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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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    1. Hi

      Trig pillars were designed long before GPS, the spider was designed for a theodolite to be attached. We haven’t used trig pillars for decades and the surveying team today use GNSS receivers on poles.

      Many thanks

  7. Lee

    one of the saddest things is the very first trig pillar is inaccessible.
    It’s about 50 meters in on some farm land and there is no access to this bit of history. The farmer even parks a trailer in front of it so you can’t see it. Real shame as it was such a significant development.

    1. Jocelyn

      Stuart, thank-you for your question. Pinpointing an exact year is hard unfortunately. Using the traditional survey techniques for which they were designed (using a theodolite), trig pillars have been redundant since the mid 90s. Once used with GPS, trig pillars have been fully redundant from all uses since roughly 2001. I hope this helps, Jocelyn

  8. Jon Glew

    Trig pillars may be redundant for OS surveying purposes, but over 300 of them, along with nearly 700 other triangulation stations (surface blocks, bolts, FBMs, FBM auxiliaries etc) form the OS’s Passive Network which is supposedly still maintained by the OS and is available to third-party surveyors as and when needed.

  9. stuart williams


    When Trig’ points and OS bench marked were being used and maintained as a height datum how accurate were they relative to their start point in Cornwall?


    1. Jocelyn

      Hi Stuart.
      Levelling Networks are never given an accuracy relative to a single point/datum, instead they are expressed as an accuracy relative to distance levelled for example ±2.0√km mm. Like all other traditional control networks, the levelling network is broken down into smaller parts of decreasing accuracy and increasing density. So, at the top of the hierarchy we have the approx. 200 Fundamental Bench Marks (FBMs) which are very accurate and can be considered as “zero order” and almost error free. The FBMs still form the realisation of Ordnance Datum Newlyn (ODN) – their heights are a crucial input to the OSGM15 geoid model that models the relationship between ODN and heights from GPS receivers.
      The relative accuracy of lower order benchmarks, including the familiar cut marks, can be expressed as a function of the distance levelled. Details are given in the “Benchmarks” section on this web page which details the accuracy of our legacy control data sets – https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/accuracy-legacy-data.html
      Hope this helps, Jocelyn

    1. Jocelyn

      The instrument testing scales look to be an instrument to check the horizontal scale on a theodolite (ie for measuring angles not weight) but we cannot be certain, my colleague I spoke to says he has not used anything similar. The three equidistant grooves look like the top of a pillar and would ‘force centre’ a tribrach, but it doesn’t in itself have anything to do with trigs. Hope this helps, Jocelyn

      1. John Banfield

        Hi Jocelyn

        So a check horizontal scale to check the theodolite’s horizontal rotational movement. That’s interesting. Thank you for the explanation. I bought a periscope sextant from the same place. I do know what that does so I have a chance of using that. Think I will leave the instrument testing scale alone.

        Many thanks


  10. Peter Marsh

    Can you please explaine the function of the four small recesses in the flush
    bracket above the bench mark arrow. I have an old and vague recollection
    of them locating a crab like device which then protrudes away from the pillar
    at the height of the bench mark. Is this correct?

    1. Hi Peter

      Yes, you’re right, the recesses were designed to support a bracket which was used when the benchmark was being observed. Obviously, these haven’t been used for decades now and we use modern GNSS equipment for surveying today.

      Many thanks

  11. Peter Marsh


    Thanks for previous reply 22 Oct.

    Can you explain a little more fully how this bracket was used.

    A diagram would be very helpful.


    1. Jocelyn

      Peter, thanks for your question. This has been passed on and we will get back to you as soon as we can. Many thanks, Jocelyn


    Good afternoon, i am a member of the Cunningsburgh History Group In the Shetland Islands, there is a trig point at Helliness Cunningsburgh that over looks the sea where on the 30th June 1917 a destroyer HMS Cheerful that was escorting a convoy of nine ships hit a contact mine she broke in two and sunk, 44 men lost their lives, this event has more or less been forgotten about, we did remember them on the 100th anniversary and led a wreath at the trig point, we intend to research this as a project try and get contact with decendents of these men, and we would like to remember these men and thought if we could commission a plaque with the names of these men and the story of the ship and wondered if it was possible to erect the plaques on your trig point, this would be a nice way to reuse your trig point and would be a focal point for people and decendents to come and pay there respects to this tragic event that happened over 100 year ago, thanks for your time and hope to hear from you Ian.

    1. Jocelyn

      Hi Ian, thanks for your comment. We have no problem with a plaque as long as it does not interfere with the top plate of the pillar or the flush bracket on the side. We recommend using exterior grade adhesive to fix to the pillar rather than bolts. This will reduce the risk of water ingress to the concrete and eventual frost fracturing. You will need land owner permission to access the pillar to carry out the work and we recommend you also check with them that they are okay with more people being likely to want to visit the pillar once the plaque is attached. Hope this helps, Jocelyn

  13. Hello, I have recently started working in a 1960’s housing estate in Birmingham and was thrilled to find a trig point while exploring the other day. Would you have any records as to when it was installed etc or what number it was? It’s approx 52.404862 -1.893424
    Many thanks

    1. Jocelyn

      Hi Jayne. Based on the lat and long you gave, my colleague is pretty sure you’re referring to the “Bells Lane” pillar at map ref SP073785. If you search km square reference SP0778 on our website’s trig locator tool (https://www.ordnancesurvey.co.uk/gps/legacy-control-information/triangulation-stations), that will provide some information. We don’t hold accessible info on when it was built, but my colleague has advised me it looks like an early-ish one as it was computed in 1944. Hope this helps, Jocelyn

  14. Christian

    Hi Gemma,

    I’ve been bagging Trig points for sometime now and I’m trying to purchase a Theodolite to fit them. Was there a specific make and model used by Ordnance Survey please?

    1. Jocelyn

      Christian, thank-you for your question. OS has used a wide variety of theodolites in their duties. The exact make and model is not important if the only requirement is to ‘fit’ a trig pillar. A trig pillar adaptor is used to fit between the theodolite and the three metal groves set into the top of the trig pillar (the ‘spider’). This adaptor consists of a metal plate with three pins on the underside which locate into the grooves on the trig pillar, and a 5/8” survey thread in the centre which screws into the base of most theodolites.
      You can purchase a trig pillar adaptor from survey supply companies but these can be quite expensive. Those used by the OS are quite simple and could be quite easily made (a circular plate of metal with three legs set at 120 degrees around the bottom edge, and a 5/8” survey thread in the centre pointing upwards).
      We may be able to source a photo too if you are interested, but unfortunately we cannot upload one here in a comment. If you would like a photo, please email me at joss.harris@os.uk. Many thanks, Jocelyn

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  16. Geoff

    Hi, what do the numbers on the trig point refer to? The one nearest to me has S4014 on it. Is this just its number of the 6500 constructed or does it refer to something else?
    It is in Sompting, West Sussex.
    Many thanks in advance.

    1. Jocelyn

      Hi Geoff, good question! S4014 is the flush bracket number. The numbers are simply a unique identifying number cast into the bracket when it was made. There is no direct relationship between the number and the location, ie number S4014 would have been the 4014th cast in the ‘S’ series of flush brackets but not necessarily (and almost certainly) not the 4014th trig pillar built. Some flush brackets are prefixed with a ‘G’ and these tend to be first order Geodetic Levelling monuments. Most trig pillars (but not all) are prefixed with an ‘S’. Some flush brackets have only a number cast into it with no prefix letter. I hope this helps, Jocelyn

  17. John Nichol

    Good morning
    Trig point 3477 Burtonwood near Warrington is in the grass verge of Clay Lane. Just to let you know that Warrington Borough council will be constructing a new footway next to Clay Lane in the next 12 months. The trig point will not be affected as it will remain within the grass verge. We would like to put a plaque on the pillar to mark the opening of the new path. Who do we notify at the OS?

  18. Martin Lewis


    Just going back to the question raised by Peter Marsh in October 2018.

    “Can you please explain the function of the four small recesses in the flush
    bracket above the bench mark arrow. I have an old and vague recollection
    of them locating a crab like device which then protrudes away from the pillar
    at the height of the bench mark. Is this correct?”

    Would it be possible to have a diagram of the benchmark bracket if it is still available please?

    1. Jocelyn

      Thanks for your interest Martin. I will forward you the image now to the email address you have linked to WordPress. In case it goes to your junk, it will be from joss.harris@os.uk. Many thanks, Jocelyn

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  21. Robert Elsden

    Can you offer more information about the actual observations that were made from the pillars? As a surveyor I would like to know a bit about the process of the observations. I’m guessing that they were done at night when the atmospheric conditions were better. How were the targets illuminated?, how many targets were observed in each instance etc?

    1. Hi Robert

      Even better, “A history of the retriangulation of great Britain” has all the detail that you could ever need about the observations and everything else that was done for the retriangulation – which is the original purpose of our trig pillars. All the observations are listed, and the entire book is scanned and available on the OS website. Here is the link – https://www.ordnancesurvey.co.uk/docs/ebooks/history-retriangulation-great-britain-1935-1962.pdf (please note it is 150MB).

      Many thanks

      1. Abi

        Good evening

        I’ve tried this link but it seems to be broken. I’m keen to do some research into trig points in Sussex particularly on the South Downs Way. My sister tells me that my Grandpa was stationed here in Brighton during WWII and one of his jobs was to work on the trig points round here. I’d love to know if this was true and what work he did. Any help or pointers would be gratefully received.

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  24. Hi

    Ward’s Stone near to me here in Lancashire has two trig pillars on it. I’ve always had a love for trig pillars and have not seen a hill with two anywhere else in my hill walking. I imagine there are others. How many other multiple trig point hills are there in the UK please? I’m interested for an article I am writing.

    Kind regards

  25. Jonathan Kington

    I was wondering if you know of anywhere I could obtain information on the 19th century trigonometrical points that predate trig pillars. I believe there is one near me in Wigtownshire but I cannot be sure. There is one marked on the 1850 6inch map as a triangle with a dot, the mark on the ground is on living rock in the form of a compass and has three slots which appear to be for a tripod. By the way I am a benchmark “nerd”.

    1. Jocelyn

      Jonathan, thanks for your question. We haven’t held historic archives since completing the move to our current HQ back in 2011. We do have the location of a number of trig pillars (as shown in OS Maps) but you would need to go to one of our historic custodians for research especially as its 1850. Please try National Archives (https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/ordnance-survey/), National Library of Scotland (https://www.nls.uk/collections/maps) or the British Library (https://www.bl.uk/catalogues-and-collections). Hope this helps, Jocelyn.

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  27. Phil Johnstone

    I have two questions:
    1. OS maps that pre-date the 1936-62 retriangulation bear symbols identical to those used for trig points (i.e. a triangle with a dot in it). What sort of marker do these symbols refer to?
    2. Do you have any information on the ‘concrete ring’ markers that are found in Northern England? 27 are known but more must be now buried beneath topsoil. The most obvious one is on the summit of Blencathra in the Lake District. They are presumably accurately surveyed, but the one on the Cross Fell plateau (NGR NY691342) is not even marked by a spot height on the maps.

    1. Jocelyn

      Phil, thanks so much for your question. Our response includes a few images so I will email you our answer to the email address you’ve used for your account. I hope that is okay? If it does not arrive please make sure you check your junk folder. Many thanks, Jocelyn

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    1. Jocelyn

      Martin, we are not aware that any particular orientation was chosen for the spider during construction. Hope this helps, Jocelyn

  29. Allan K

    Hi, thank to all for this very informative page! I have been “trig bagging” in south east Scotland this year and have had some wonderful experiences. However yesterday we were prevented from accessing the Primary trig pillar at Lumsdaine (OS Grid Ref: NT 872 683) where we found the site has been deliberately surrounded by barbed wire and the landowners confronted us aggressively.

    Can you advise on the requirement to permit access to these important historical monuments please. Bearing in mind the Scottish Outdoor Access Code, we have reported this to the local Council and we would like to know the Ordnance’s position on this please.

    Cheers, Allan

  30. Mike Misitirii

    What treasure is beneath a trig pillar that many have been vandalized??one had been put up in my ancestral land and to date some people still seek & dig where it had been erected..

    1. Jocelyn

      How interesting Mike! As much as we’d love it to be true, as far as we know there is no treasure. Many thanks, Jocelyn

  31. Informative piece!! Understanding the principle of triangulation is the biggest learning revolution of our times. It truly makes the learner take charge of his/her career.

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