Keeping our fundamental bench marks in order

Guest blog by Colin Fane, Geodesy and Positioning Consultant

Lampeter fundamental bench mark

Most people are familiar with trig pillars. There are over 7,000 of them scattered around Great Britain. However, there is another, considerably more elusive, type of pillar to be seen across the land – the fundamental bench mark (FBM). FBMs are the physical realisation of our national height datum ‘Ordnance Datum Newlyn’ (ODN – mean sea level at Newlyn, Cornwall, 1915-1921) and are still crucial in defining this reference system today.

There are nearly 200 FBMs around Great Britain, mostly constructed in the first half of the twentieth century at sites carefully selected to provide an anchor to bedrock. 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 approximately three-quarters of a million bench marks, using less precise levelling. These lower-order bench marks are often seen cut into stone at the base of a building, church or bridge and about half a million of them are still in existence today

Approximately half the FBMs are surrounded by railings, particularly in urban areas. To the casual observer the railings could be mistaken for a receptacle for other people’s rubbish (or at least this seems to be what some people use them for). However, the railings do provide protection and some of those FBMs without them can get damaged.

Lampeter FBM (SN5752) is one such example. It was brought to our attention that the pillar had been broken in two and the visible section of the pillar, above ground, was now laying on the ground. As these monuments are still important to our work, it was essential to rescue the broken pillar (before someone liberated it as a garden ornament!) and to repair it.

Lampeter FBM fully restored

The visible section of an FBM pillar is usually about nine by 11 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’.

The broken part of the Lampeter pillar was repaired using a high-tech resin (in a delicate shade of bubble-gum pink) which was allowed to harden for 24 hours. We then re-heighted the top bolt on the pillar to determine its new altitude relative to ODN.  This was done with a modern digital level – capable of measuring a difference in height to a precision of 0.00001 metres.

While we were working on the site, locals informed us that a Spitfire had crashed there during World War II, but we didn’t find any sign of it. Lampeter FBM is the first FBM we’ve had to re-height in over 20 years – so who knows when we’ll be working on one again?

13 Responses

  1. FBMs are actually quite a popular bagging thing for trig enthusiasts! There’s something oddly satisfying about them and nice to see one getting repaired. On a local note, does the OS have any records/photos about the Govan FBM in Glasgow? Now since replaced by an ASDA carpark…..


  2. Bob Funnell

    I realise the above blog was some 4 years ago but hope I can gleen some info.

    Torrington Fundamental Bench Mark, Grid Ref: SS4855 1938 has been unloved for a number of years. During my work on the Commons I have managed to keep the weeds and grass at bay. Photos can be found at: http://trigpointing.uk/trig/7225. I would like to see it rejuvenated!

    My question is: Who is responsible for the upkeep of this piece of history; please?

    1. Hi Bob

      OS retains responsibility for all trig pillars and fundamental benchmarks and if they’re damaged or become unsafe to the public we’ll step in. Although this one looks a little unloved, it doesn’t look like the team need to carry out repairs on it. We’ll pass it along to the team along with your feedback though.

      Many thanks

  3. Katherine Anteney

    We would like to know the difference between a trig pillar and an FBM. We recently bagged Giant’s Grave trig pillar in Wiltshire and there is what we assume is an FBM right next to it. Why would they be so close together… what do they measure differently.

    1. Hi Katherine

      Good question. An FBM is there to specifically measure height and is still an important part of OS today, as it says in this blog.

      Trig pillars were used as bases to help map out the whole country – using triangulation to work out a location in relation to another object. They were designed for a surveyor to place a theodolite on top (fixed to the brass plate you see on top of each trig) and take measurements. When built, a surveyor would have been able to see at least two other trig pillars from his position, using the theodolite, and this enabled OS to map the whole country. Trig pillars have long since been replaced by newer technologies and our surveyors use GNSS equipment and laptops to carry out their positioning and measurements now.

      Many thanks

    1. Hi Ian

      It varies as benchmarks are carved into a variety of places – from private property to publicly accessible buildings such as churches. There are around half a million (steadily reducing as developments take places across Britain), so there’s still a good chance of spotting one in most towns. Our benchmark locator is here: https://www.ordnancesurvey.co.uk/benchmarks/

      Many thanks, Gemma

  4. Rebecca Hiorns

    Hi Gemma,
    We are working on a Heritage Lottery funded project to restore Great Linford Manor Park, Milton Keynes. It contains a Fundamental Bench Mark which we would like to provide interpretation of so people can understand its purpose. I have found your blog very helpful I am particularly interested that a Fundamental Bench Mark is not a relic on past map making, but is still needed today. Could you explain more about how they are still important today?
    Many Thanks

    1. Hi Rebecca

      The FBMs are 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.

      I hope this helps.

      Kind regards

      1. Rebecca Hiorns

        Thanks Gemma – I have read that when Fundamental Bench Marks were constructed they were intended to help monitor vertical movements in the earth’s crust. Are they still used for this purpose today?

  5. S.Wright

    Hi, as a surveyor of over 30 years I had the sense to save a unusual Brass Bench Mark Plate. It measures 230 mm wide by 153 mm deep and was set into the corner of a factory complex in Birmingham. It has the height marked at 310.00 feet and has the words Newlyn Datum on the bottom of the casting. It is quite heavy and beautifully made. Is it a private plate or was it put there by Ordnance Survey?

  6. Pingback : Benchmark Bagging – Blog

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