It may not be Mont Blanc but every little helps: Britain’s highest mountain is now officially a metre taller. So what happened, and how do the Ordnance Survey work out how high places are anyway?

Back in 1949, a team of OS surveyors spent 20 consecutive nights hauling 90 kilos of equipment up Ben Nevis and surrounding summits to take the best measurements possible with the equipment of the time. They had to work at night, because they collected data by shining strong lights from the trig pillars of other mountains onto the one on Ben Nevis. It’s the last time the mountain was surveyed.

Then, when the cairn on Ben Nevis’s summit was repaired recently, OS surveyors were asked to check out what had been done and took the opportunity to remeasure the mountain. This time a team of three headed up for a day in the hills, armed with waterproofs, sandwiches, the latest geodetic survey-grade GPS receiver and a laptop.

It was a classic Scottish mix of raining, sleeting and snowing at the summit, but harsh weather doesn’t affect the OS’s modern equipment or readings. The receivers they use nowadays can simultaneously track 20-30 satellites every second, which are some 20,000km away in the sky, travelling at several thousand kilometres per hour.

The surveyors simply put this receiver on the summit of Ben Nevis, let it communicate with satellites orbiting the earth for two hours, ate their sandwiches, then headed back to OS head office, where the real hard work begins these days. It’s not simple – in fact it was nearly two weeks before they had confirmed how how high Ben Nevis is.

109 base stations around the country provide a reference framework for the OS, and the surveyors began by checking the position of the readings taken on Ben Nevis relative to these.

Then certain accurate orbits and earth rotation models were gathered from scientific institutions around the world, and these and a bit of maths were used to get the best possible height answer. It’s all designed to model out all the possible errors that might be in the GPS signal, caused by atmosphere, tropsophere, ionosphere; all the things that contribute to making the signal a bit fuzzy.

Mark Greaves, Ordnance Survey’s geodetic consultant, commented: “The new height was measured as 1344.527m. The measured height has changed by centimetres, but those centimetres mean we now need to round up rather than down. So that’s why Ben Nevis will now be officially known as 1,345m.”

“What is amazing is how close the surveyors in 1949 were. It took them 20 nights, because they only had three clear nights in that period to get it right. To do the best possible job it had to be run with military precision; everything they did had to be timed to perfection. Their effort and accuracy is remarkable.”

Ben Nevis actually has two heights: the new measurement is for the highest natural point on the summit. The cairn is slightly lower, which is why Ben Nevis is recorded on current maps as 1,343m (1,344m). Users of OS’s digital map service OS Maps will see the height change immediately; paper maps will be changed forthwith.

Did you know

250 OS surveyors are out mapping the UK every day. Over 10,000 changes take place each day in what is thought to be the largest geospatial database of its kind in the world. The OS holds up to 460 million geographic features in its database of Great Britain.

WATCH Ben Nevis 1345m: The Maths

WATCH Ben Nevis 1,345m: The Trip

WATCH Ben Nevis 1,345m: Another Age

Explore Ben Nevis in virtual reality

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