The hill sleuths on Thack Moor with their GPS receiver: from left, Myrddyn Phillips, Graham Jackson and John Barnard

Hillbaggers who think they have summited all England’s mountains will have to think again.

A new 2,000-footer will join the list of hills topping the magic mountain mark after a recent survey by a trio of amateur hill sleuths.

Thack Moor in the northern Pennines is 2,000 feet high – just.

Measurements by John Barnard, Graham Jackson and Myrddyn Phillips using high-tech GPS equipment show the otherwise unprepossessing grassy Cumbrian hill tops out above the official mountain height, by less than an inch.

Ordnance Survey, the Government mapping agency, confirmed the data collected by G&J Surveys, as the trio is known, on two visits to the hill put Thack Moor at 609.62m, just two centimetres above the metric equivalent of 2,000ft.

The authors of hill lists have agreed to amend their books to reflect the addition, and Ordnance Survey said it will amend its maps to record Thack Moor’s height as 610m.

Although amateurs, G&J Surveys’ trio of hillwalkers use the most up-to-date satellite equipment to collect height data from American Global Positioning Satellites. The data is then run through sophisticated software by Ordnance Survey to ensure maximum accuracy.

Mark Greaves, an OS geodetic analyst, said: “Essentially when they measure a hill or mountain and their result contradicts what’s on our map they send me their GPS data to check and re-process.

“I run the data through some very high end scientific GPS processing software called Bernese.

“The same software and analysis techniques are used to compute the coordinates of our OS Net stations. Given a suitable amount of data the software can compute GPS station coordinates to 2mm in plan and 6mm in height. OS Net is the national positioning infrastructure.”

The trio made two separate visits to Thack Moor when it became obvious the result was going to be so close.

Mr Greaves said: “For the Thack Moor surveys this external check showed that both times the height was accurate enough to be confident that it really was just over 2,000ft.

“The fact that the result is so close to 2,000ft is what prompted to me to ask for a second survey of the hill.

“Now two independent surveys using high quality GPS data, from different equipment, and the best possible computation and analysis have both shown that Thack Moor is indeed just over 2,000ft.

“The combined accuracy of the height measurement from both surveys was plus or minus 2cm so, even erring on the side of caution, it is still 2,000ft and it’s equally likely it could be 2,000ft and 4cm.

The surveyors pinpoint the highest part of the hill, which is partly covered in snow

“Our cartography department have been informed of the change at Thack Moor – the height goes from 609 to 610 – and we hope to have the change in our digital data soon.”

Thack Moor will lose out in one respect: it will no longer be classed as a Dewey, an English, Welsh or Manx hill between 500m and 609.6m in height with a minimum of 30m of drop in a list compiled by Michael Dewey.

It will, however, be added to two lists in the complicated world of hill heights. The Cumbrian moor will become a Hewitt – the acronym for Hill in England, Wales or Ireland over Two Thousand feet which must have a minimum prominence of 30m. List keeper Alan Dawson has agreed to add the hill to his compilation.

And Anne and John Nuttall, keepers of a set of hills with the slightly different criteria of 2,000ft height and 15m drop, also said they will add the hill to their Nuttall list.

Anne Nuttall revealed the pair have already made it to the summit of Thack Moor.

John Nuttall on Thack Moor during his and Anne Nuttall's north Pennines journey. Photo: Anne Nuttall

She said: “John and I have already ticked Thack Moor off – well in advance of Graham, John and Myrddyn.

“We didn’t know it was a two-thousander then of course, but in May 1998 we did a north Pennines backpack.

“We started from Appleby and walked via Dufton up High Cup Nick and camped on Backstone Edge.

“On day two we continued over Knock Fell, Great and Little Dun Fells, Cross Fell, Melmerby Fell and Fiends Fell and camped on the summit of Black Fell.

“Day three was over Thack Moor then on to Croglin and to Armathwaite and back by train.”

The trio of amateur surveyors have worked with Julia Bradbury to find Wiltshire’s highest point; elevated Glyder Fawr to 1,000m status, and demoted Sgurr nan Ceannaichean from the munro list.