Pedantic Press Release March 2023
Most mountain heights are wrong
Recent research has shown that over half of the mountains in Britain have the wrong height shown on Ordnance Survey maps and in hundreds of guidebooks. A ten-year programme of landscape surveying using satellite technology (GNSS, often known as GPS) has measured the heights of over two thousand summits more accurately than ever before. Analysis has shown that over half of the summit heights on current maps are wrong, though not by much. For example, the latest edition of The Munros guidebook published by the Scottish Mountaineering Trust lists 508 summits over 3000 feet high. Over 80% of these (432) have now been surveyed using GNSS (Global Navigation Satellite System). The data shows that 58% of the published heights are wrong by one metre or more, but 97% are correct to within three metres.
Full details of survey findings from the higher summits are available in a book published in 2021 by Pedantic Press: The 1033 High Hills of Britain, by Alan Dawson. The book includes numerous stories and photographs to describe and illustrate climbing and surveying the hills, as well as the relevant survey data.
It might seem unlikely that one man in his sixties could obtain far more accurate results than Ordnance Survey or the Scottish Mountaineering Club, but this is in fact the case. The reason is that Ordnance Survey uses aerial surveys - taking photographs from planes - to obtain most mountain heights. This method is effective for OS purposes as it allows maps of the whole country to be produced in an efficient way. Satellite-based surveying is more labour intensive, as it requires carrying heavy survey equipment to the top of each summit and waiting there to record satellite data. However, it is forty to fifty times more accurate for obtaining heights of summits or any specific points of interest. Aerial surveys give heights that are accurate to within three metres, compared to six centimetres for GNSS surveying.
Surveying the summit of Sgurr Dubh na Da Bheinn (938.6m) on Skye in May 2017
Notes
- Most spot heights on Ordnance Survey maps are derived from aerial photography and photogrammetry, but heights of triangulation points are derived from ground-based surveys and are accurate to within one metre.
- Heights on OS maps are rounded to the nearest metre. Where a GNSS survey result differs from a map height by more than half a metre, this makes the rounded figure wrong, although it may differ from the actual height by less than one metre.
- GNSS surveying makes use of satellite data recorded by the OS Net network of base stations, run by Ordnance Survey. In conjunction with suitable equipment, software and expertise, this enables the heights of hills or other features to be measured to within six centimetres of accuracy, subject to conditions on the day of the survey and the margin for error on the ground, which varies according to the nature of the terrain.
- The largest height error found so far on an OS map is 34 metres. The summit of Ceum na h-Aon-choise in Kintail is shown as 891m but the actual height is 924.9m (3034 feet). This peak was not listed by Hugh Munro as he could not have known that it is over 3000 feet high. The OS map figure was due to cartographic error.
- Rocky summits can be measured more accurately than vegetated summits. The biggest problem for hill surveying is not the weather or the technology but the large cairns built by walkers on the top of numerous hills. These may obscure the highest point and can interfere with satellite signals, so they limit the level of accuracy that can be obtained. Smaller cairns can be removed or replaced. Large cairns are also likely to be the cause of some map heights being wrong, as it can be difficult to distinguish cairns from rocks with aerial photography. For example, the OS height of Gulvain in the Western Highlands is 987m, but the base of the huge summit cairn is only 983.2m. It is not unusual for older OS maps to show more accurate heights than recent maps, as heights on older maps were derived from ground surveys.
- For most purposes, OS maps are superbly useful and accurate. However, numerous hills have different heights on different scales and editions of OS maps, as they have different sources. OS are now trying to make their maps more consistent, though that does not necessarily mean more accurate.
- All survey reports are gradually being added to the Pedantic website at pedantic.org.uk/reports. This enables anyone interested or sceptical to check the consistency of height figures obtained with reference to various OS Net base stations, as well as the duration of data collection and other relevant information. Details are added to the authoritative Database of British and Irish Hills as the reports are published.
- Initial analysis of survey results from lower hills has shown that about half of summit heights are wrong, but precise comparison between maps and survey results is difficult, because many summits have different heights on different scales of map, and some hills have no height shown at either 1:25000 or 1:50000 scale.
- There are limits to pedantic precision. In August 2022, a GNSS survey on the north face of Ben Nevis revealed that the Douglas Boulder at the base of Tower Ridge is 973.5m high and has sufficient relative height for it to qualify as the 1034th summit eligible for the list of The 1033 High Hills of Britain. You can't win them all, but you can climb them all. By early 2023 only three people had done that, but lots more had climbed over 900.
Surveying the summit of Meall Glas (959.3m) in January 2017
Background
Alan Dawson was born in England but moved to Scotland in 1989 as it has a lot more hills. He began surveying hills after retiring from his job with the University of Strathclyde. He now lives in a small village in the Highlands surrounded by lots of hills and not many people. He is a member of The Munro Society and Mountaineering Scotland. He is believed to have surveyed more hills using GNSS equipment than anyone else in the world.
Further information
Pedantic Press: pedantic.org.uk
The 1033 High Hills of Britain: pedantic.org.uk/books
Database of British and Irish Hills and Hill Bagging
How Accurate Was Munro? pedantic.org.uk/articles
Surveying and Mapping Standards: pedantic.org.uk/articles
Pedantic survey reports: pedantic.org.uk/reports
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