Rising sea levels could topple the proud status of some British mountains, reducing them to the status of mere hills. Those in greatest danger of demotion include a peak in the Yorkshire Dales that was only reclassified as a mountain a few weeks ago.

The Ordnance Survey (OS) uses mean sea level as the starting point for measuring the absolute height of mountains, which must be a at least 609.6 metres (2,000ft) above sea level – but several peaks in England, Scotland and Wales are only a few centimetres taller than that.

Mean sea level, the halfway mark between high and low tides, is measured by a gauge at Newlyn in Cornwall. But the point used by the OS was established almost a century ago, and since then sea levels have risen and are continuing to rise at an accelerating rate, mainly through climate change.

“We have to measure from a fixed point, and there are no immediate proposals for a change, but rising sea levels could obviously be a factor if there is a change in the future,” an OS spokesman said. “Clearly if the fixed point was taken from a higher level, the heights measured would drop by the same amount, and that certainly could affect many hills and mountains.”

Thack Moor in Cumbria, which was only recently reclassified as a mountain. Photograph: Andy Sutton/Alamy Stock Photo

Calf Top in the Yorkshire Dales only achieved mountain status in September, after the OS determined its true height is a few centimetres taller than was concluded when it was last measured in 2010, putting it just over the required height for a mountain. Any change in the base point would knock it back down again.

Myrddyn Phillips is an amateur surveyor who worked on Calf Top with Hill Data and Mountain Surveys, one of several amateur groups whose hours of unpaid work, out on the slopes in all weathers, has produced new data that has been accepted by the authorities.

Thack Moor in Cumbria also recently became a mountain through Phillips’s work with John Barnard and Graham Jackson, of G&J Surveys. Their measurements, repeated at the request of the OS, confirmed its true height was just 2cm over the 609.6-metre qualifier.



Phillips was enthusiastic about the possibility of change: “That would be fantastic, on the whole I like change.

“It will be incredibly interesting if Ordnance Survey change their datum point as this will affect all known heights throughout Britain, and even if this change equates to only 40-45cm it will affect a multitude of hill and mountain classifications,” he said.



Looking towards Tryfan in Snowdonia national park, north Wales. The hill is part of the Nuttall group of hills. Photograph: Alamy Stock Photo

A change could affect the status of many sites cherished by walkers who like to tackle all the mountains in a group, including the Munros in Scotland, which are all over 914 metres (3,000ft), the slightly less challenging 762-metre (2,500ft) Corbetts, and the Hewitts, Nuttalls and Deweys in England and Wales.

“In a general sense it does not matter whether a peak is classified as a hill, mountain, Munro or a Pedwar [a hill in Wales between 400-499 metres], but we do seem to have an inner need to categorise and this is as applicable to hills as any other form of commodity,” Phillips said.

“Certainly for the hill bagging community it is essential to get hill classifications correct, and it is also justifiable to say this in relation to the wider hill walking community as there are upwards of 6,000 people who have now completed the Munros and probably twice as many working their way through this list, and all of these people will want to know that they have completed each and every Munro that should be listed.”

He acknowledged that many people would be upset if their cherished mountain view was downgraded but insisted it would be an exciting process.

“As I am a person who is open to change this would also cause a lot of fun as there would be a Munro reclassified to Corbett status, a Corbett reclassified to Graham status, Simms would disappear, Hewitts and Nuttals would become Deweys, a Dewey would become a Pedwar and a number of Pedwarau would become 300m Twmpau hills.”