The Welshmen who went up a hill and came down a mountain

If it sounds like it could be the plot to a movie, that's because it was. Thirteen years ago, Hugh Grant starred in The Englishman Who Went Up a Hill But Came Down a Mountain, a comedy directed by Chris Monger about two English cartographers who try to downgrade a Welsh mountain to a hill.

In the film, set in 1917, villagers fought to ensure that the "first mountain inside Wales" was kept exactly that, despite the best efforts of Grant and his sidekick to change its status before heading back to England.

Now Monger's film has come to life – albeit in reverse – although Grant is nowhere to be seen. Walkers in Snowdonia have converted a hill into a mountain after taking their own measurements of one of their favourite peaks.

Mynydd Graig Goch was previously recorded as peaking at 1,998ft (609m), just two feet shorter than the required height to tip a hill into the category of mountain. But now three ramblers have measured the muddy peak at six inches over 2,000ft, challenging previous Ordnance Survey measurements and precipitating a formal reclassification.

Before their reassessment, Wales had three hills listed as being 1,998ft: Mynydd Graig Goch and Craig Fach, both in Snowdonia, and Mynydd Troed near Crickhowell, in Powys, central Wales.

A group of keen walkers, Myrddyn Phillips, Graham Jackson and John Barnard, were confident that Mynydd Troed's status was correct but they had their doubts about the other two. So, using equipment specially manufactured by the Swiss firm Leica Geosystems, the trio used satellite positioning technology to gauge the exact height of the Snowdonia hills to the nearest inch.

Their survey confirmed that Craig Fach was indeed a hill, standing at 1,997ft. But further research showed that Mynydd Graig Goch was six inches over 2,000ft. It is now hoped that Ordnance Survey will update its records, and in so doing bring the total number of mountains in Wales to 190.

"It's fantastic. Nothing like this had happened before," said Mr Phillips, from Welshpool, Powys. "We're very pleased our survey has proved that Mynydd Graig Goch is a mountain and not a hill. Ordnance Survey has agreed to update its maps on the internet straight away, but it might take a bit longer to correct the paper maps."

Mr Phillips pointed out that because Ordnance Survey's spot height measurements have a margin of error of plus or minus three metres, it is hard to argue that the original measurement was wrong. Nevertheless, improved technology and measuring techniques mean the reclassification of hills into mountains – and vice versa – is likely to become increasingly prevalent.

The three ramblers, who took the measurements on 11 August and spent two hours taking more than 7,000 readings on Mynydd Graig Goch, were hampered by the rough weather common to Snowdonia for much of the year.

"Winds of 40-50 mph made things quite difficult for us and it rained, but it was worth it," said Mr Phillips.

Paul Beauchamp, of Ordnance Survey, said: "Our involvement has been really just a matter of verification. We know the company that provided the technology very well and have worked closely with them in the past. We were very pleased to tell these three enthusiasts that the hill snuck in at just 6 inches, or 15cm, over the 2,000ft threshold, making it a mountain. Our digital data will be updated over the next few weeks. Our two commercially available books, the OS Explorer and OS Landranger, are revised on a cyclical basis, and you can expect the updated version to be released over 2009-10."