Surveying Ben Nevis – the 49ers

It’s close to 170 years since OS officially confirmed Ben Nevis as Britain’s highest mountain. It’s nearly 70 years since we carried out the last full survey back in 1949. Now we know in 2016 that the surveyors of a bygone age were just centimetres out in their calculations – testament to their extraordinary efforts and application.

The type of beacons used and the surveyor’s tent for protection through the night

The type of beacons used and the surveyor’s tent for protection through the night

In 1949 theodolites and other antiquities of surveying were the best friends of those mapping every nook and cranny of the British landscape. Now highly refined measuring devices, linked to an intricate network of land stations and a sky-full of satellites (yes up to 15 Russian, American and European ones) have been used to re-check the height of the mountain taking three surveyors just two hours to gather what it took an expeditionary force of surveyors 20 days to complete.

“It’s given our current OS surveyors a new found respect for the class of 1949.” Says Mark Greaves, our top Geodetic expert.

So how did they do it?

It required a seven-man team to lug 200 lbs of kit to the top of the summit every day for three weeks, leaving at seven in the evening and returning at five in the morning. The array of equipment included surveying equipment, tents, provisions and cooking utensils and stoves. This was being mirrored on other summits surrounding the highest peak too (Ben Lomond, Ben Lawyers, Ben Cruachan, and Ben Alder).

Night-time was the surveyors best friend because the readings from their Cook and Troughton theodolite would lock on to massively powerful light beacons which other surveyors were shining at the Ben Nevis summit from the other peaks. It was the most accurate pin-point way to fix their measurements at the time.

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

It was a military exercise in co-ordination and time-keeping to ensure everyone reached their station at the right time. And how do you keep several beacons of search light intensity powered? Every night they also hefted massive car batteries (see photo) up and then down for recharging which OS surveyor hearsay suggests burnt their coats when the acid leaked.


Bernard Willis, the team leader was quoted in the papers at the time, “One one night we experienced a blizzard. Our tent was covered in snow. Last night the Ben was clear and we could pick out the beacon on Ben Lomond distinctly.”

We’ve been mapping changes in the British landscape for the last 225 years and now those changes are on a digital map, in our database of 500 million features, within weeks, which is about the same time and the same deliberation and attention which drove the class of ’49 to map Ben Nevis whatever conditions could throw at them.

Find out more about their amazing feat in our video.

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4 Responses

  1. Victor Abbott

    What technique was used to derive the height; simultaneous reciprocal vertical angles? Whichever method, from what values were the observations taken? What was the derived value and what was the computed uncertainty?

    1. Andy

      I’ve consulted our experts. The short answer is – all these fine details are lost in the mists of time or buried VERY deep in the archives. We can be 99.9% sure that the technique used was NOT simultaneous reciprocal vertical angles. Just VAs observed from Ben Nevis to other mountains and then, on different dates, back to Ben Nevis when the other mountains were observed from.

  2. Richard Cushing

    Having worked with 17th century estate plans I am in awe of the accuracy which the surveyors of the day achieved, and would always assume that an historic survey is accurate but that features have changed, unless and until it is shown otherwise.
    Could we please have the absolute height recorded in 1949, together with the height recorded in 2016, preferably with the uncertainty in both cases? Presumably these are available to a few millimetres?
    As I understand it, the OS quotes heights to a point on the mountain top, not the trig post, so are we sure that the point on the ground is the same today as it was in 1949, and that there has been no geological movement?

    1. Hi Richard

      If you take a look at the original blog on the Ben Nevis re-survey, you’ll see the new height relates to the highest natural point on the summit (which in the case of Ben Nevis, is not where the trig pillar is placed, that is on top of a man-made cairn) and was measured as 1344.527m. There’s more information here, and links to a series of videos that explain how the measurement was carried out, and why there are two different height points marked on the map: https://www.ordnancesurvey.co.uk/blog/2016/03/britains-tallest-mountain-is-taller/

      We’ve also been asked about the exact measurement from the 1949 survey, and you’ll see that we’ve replied to say that sadly, the records from the last survey in 1949 do not show the exact measurement taken, so we only know the official rounded-up figure of 1,344m which has been recorded on maps, which in many ways reflects the changes in both surveying and the recording of data between 1949 and 2016.

      Many thanks

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