Benchmark or trig pillar: what’s in a name?

We’ve had a few questions recently about benchmarks and trig pillars and what they are and how they differ, so we thought we’d clear it up.

The benchmark

Most weeks we’ll see a Twitter conversation where someone is asking what this mark is:

Many think it is War Office-related, but it is in fact an OS benchmark (BM) and a means of marking a height above sea level. Surveyors in our history made these marks to record height above Ordnance Datum Newlyn (ODN – mean sea level determined at Newlyn in Cornwall). If the exact height of one BM was known, the exact height of the next could be found by measuring the difference in heights, through a process of spirit levelling. They can be found cut into houses, churches, bridges and many other structures. There are hundreds of thousands of them dotted across Great Britain, although we no longer use them today.

The fundamental benchmark

Less common to spot is a fundamental benchmark (FBM), which as the name suggests, is one of our high-accuracy benchmarks. There are around 190 of these which are still maintained and used by us at OS. They form our primary height network and, as such, are our link to the Ordnance Datum at Newlyn and are still crucial in defining this reference system today.

The height of each FBM relative to ODN was determined by a network of precise levelling lines across the country. The levelling network was then densified with lower order benchmarks, using less precise levelling.

The visible section of an FBM pillar is usually about nine by eleven inches and around a foot tall (to use the units of measurement from the Imperial era in which they were built), with a brass bolt set into the top and a name plate declaring it to be an ‘Ordnance Survey BM’. There is also an underground chamber containing the “master” precise reference marks for the point.

The flush bracket

The flush bracket is another way of accurately defining a height above sea level and used for the more important level control points. You will find flush brackets in most trig pillars – although not all – as well as set into walls and buildings (‘flush’ with the building).

#glasmaol #glenshee #trigpoint #trigpillar #flushbracket

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The trig pillar

Identified on an OS Explorer map by a small blue triangle with a dot in the middle, a trig pillar (triangulation pillar) is a familiar sight when you’re out and about exploring Britain. Between 1936 and 1962 around 6,500 were built to form a state-of-the-art network to re-map Britain. The trig pillar provided a solid base for the theodolites used by the survey teams engaged in the retriangulation of the country, the mathematical process that made accurate map-making possible. In the same way that BMs mark an accurate height, trig pillars mark a point with an accurate horizontal position (eastings and northings coordinates). In addition, most, but not all, trig pillars also have a flush bracket, to define their height above sea level.

Find out more about the trig pillar.

How we survey Britain today

As we’ve said above, most of these means of measuring height are now redundant for national mapping. The modern equivalent to the network of trig points 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.

Find out more about how we survey today.

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26 Responses

    1. Ric Tilbe

      Hi, Gemma. I walked up Beacon Hill, Hants yesterday and photographed the trig piller FBM. How can I find out info such as height of hill, etc?

      1. Jocelyn

        Hi Ric. The height of the hill can be found by using the contours on our maps or alternatively this information is available through OS Maps app. Hope this helps, Jocelyn

  1. Robert Reid

    Is it possible to date a bench mark? We have one in our village of Duffield in Derbyshire that is of the rarer horizontal mark. It’s on the parapet of an ancient bridge carrying the A6 over the River Ecclesbourne. There is a horizontal BM on the 14C Parish Church of St Alkmund’s. Can anyone help please? Many thanks.
    Robert Reid

  2. Victor Abbott

    Hi, Gemma,
    How was the bolt in a Bolt cut Benchmark used?
    Have you pictures of the stands that were used with a Flush Bracket, and with a common cut mark?

    1. Jocelyn

      Hi Victor. I have spoken to my colleague who works in this area. He has given me an answer and the photos you requested. As I cannot attach photos here, are you happy for me to email these through to you? Thanks, Jocelyn

      1. Victor Abbott

        Dear Jocelyn,
        Thank you finding the photos. I would be glad to receive them by email. I have also located a brass Bench(?) for a Flush Bracket.

  3. Gordon Smith

    Hi Gemma,
    I work for Guide Dogs and doing the first of a number of presentations on Benchmarking next week. I’d love to use that image.
    As a former Civil Engineer, I know what a “real” Benchmark is and intended to start my presentation with an explanation of where our use of the term comes from.
    Having a dog in the picture would be great. What permissions would I need to replicate?

  4. kevin flett

    i have a facebook page to collect our local remaining benchmarks .. still quite a few left in our town ..a real treasure trail… ‘south shields benchmarks’ on facebook and use them a lot for my cgi models ‘Souths shields cgi ‘ Also on facebook …
    i treasure these marks ..such hard work behind them 🙂 a mighty achievement

  5. John Hemingway

    Dear Gemma. Could you tell me how I can find the date of when a bench mark was made. We have a horizontal mark with the three part arrow on our Church, St Mary’s the Virgin, Willingdon, East Sussex. Many thanks. John

  6. Adam Schofield

    Hi Gemma,

    What are BM’s Measured in? How can I work out the difference in height off an old OS map (1890’s!)


    1. Jocelyn

      Hi Adam. Assuming you are asking about the units, all the BMs listed on our website are in metres. However, ones shown on old maps will be in feet and are also likely to be against the Liverpool datum not Ordnance Datum Newlyn.
      We have a page on our website that estimates the difference between Liverpool and ODN at specific locations – https://www.ordnancesurvey.co.uk/gps/legacy-control-information/liverpool-to-newlyn. A km square reference is required to calculate the difference for that area. Hope this helps, Jocelyn

  7. Tim Matthews

    Hi Gemma / Jocelyn

    Near where I live there’s a small concrete post with a benchmark symbol on it, below which reads “4½ FT”, presumably “feet”.
    Do you know what the 4½ feet might refer to?
    I can provide a photo if required.

    Many thanks,

      1. Jocelyn

        Tim, thanks for your question. Prior to seeing the image, we thought it was likely to be a boundary marker post for another government agency but not OS (many government agencies have used the broad arrow symbol) and the 4 ½ feet would be the distance from the post to the boundary. However now we have sight of it, it looks like a GPO cable marker. We suspect the GPO initials would have been at the top but have since been destroyed. See here for other examples (including some with the same ‘broad arrow’ marking): https://hiveminer.com/Tags/concrete%2Cgpo (please not we take no responsibility for 3rd party websites). Hope this helps, Jocelyn

  8. Tim

    Hi Jocelyn,

    Thanks so much for that – fascinating stuff. Very grateful 🙂

    And whilst trying a little research myself I’ve become very interested in my local benchmarks, so thanks for that too – and wish me luck visiting them all!!


  9. Keith Mortimer

    I have downloaded the benchmark locator zip file but I cannot open it. I am using the OpenOffice wordprocessing suite and when I try to open the file, the error message “Compressed (zipped) Folders Error. Windows cannot complete the extraction. The destination file could not be created.” Any ideas?

    1. Hi Keith

      Thanks for getting in touch and sorry to hear that you’re having problems. It may be an issue with the software you’re using to open the file, it does open in Excel for example and it would be worth trying that. If that’s not possible, an alternative is to view the locations in OS Maps. Without having to login, under ‘places’ you can select trig points, benchmarks etc and view them the location on the basic mapping. Here is the link for OS Maps:


      Many thanks

  10. Elizabeth Street-Thompson

    Hi Gemma,

    I have a benchmark on the side of my house (I had always wondered what it was)!

    Keen to know more of the history – how were the benchmark locations selected? Most in my area seem to be on public/non-residential buildings, but my house seems to have made the cut. I assume whoever owned the property at the time would have had to have consented?

    Look forward to hearing from you.

    1. Jocelyn

      Elizabeth, benchmarks were installed, and maintained, at different densities (x no. benchmarks per km square) depending on urban or rural. Locations were chosen to be easily accessible and on structures which were unlikely to change/be destroyed. As such churches, bridges, marker posts etc were favourite choices, but there are certainly a great many located on private dwellings. Permission would definitely have been sought before installing any benchmark. Hope this helps, Jocelyn

  11. Bob Foale

    Hi Gemma.
    I’ve downloaded the .csv file for the benchmarks, but was trying to understand what all te abbreviations mean, especially ANG?
    Can you please offer any advise or tell me where to look.
    Much appreciated.

    1. Jocelyn

      Bob, thanks for your interest. Your query is similar to one we’ve had recently so, whilst there may be a bit more information than you asked for, I think you’ll find the full response of interest. As it includes a couple of images, I will email our reply over to you via the email address you have used for your WordPress account – I hope this is okay? For your reference, the email address it will be coming from is joss.harris@os.uk. Many thanks, Jocelyn.

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