How to measure the height of a mountain

An idiot’s guide to topographic prominence

Once upon a time paid employees of the British Empire went to extraordinary lengths to calculate the height of the Himalayas. It’s quite widely known that the highest point on the planet is named not after a Tibetan mountain deity, but the one-time Surveyor-General of India, Sir George Everest. Perhaps less familiar is that in the 19th century a pundit wasn’t someone who sat on a couch in a TV studio talking rubbish about football, but was an extremely brave native Indian who risked his life wandering through the Himalayas disguised as a pilgrim secretly mapping unexplored territory by counting prayer beads.

Things are easier now. These days as long as you can get to the top of a mountain, you can just get your GPS out and discover what altitude you’re at. Measuring the height of a mountain has therefore become much simpler – or has it?

A popular pastime among hill walkers here in the UK is Munro bagging: climbing all 277 mountains in Scotland over 3000 feet in height (“Munros”). Or is it 284? No, in fact it’s currently 283, but it’ll probably change again. The reason isn’t because the land keeps moving up and down, but that people can’t make up their minds which peaks qualify as Munros. There are plenty of them more than 3000 feet high that don’t. When I first started Munro bagging myself about 15 years ago there were 284. Now there are only 283, and I don’t know whether I’ve climbed 73 or 74 because I’ve never bothered to find out if the one they docked is one of the ones I’ve climbed already. Some people spend 40 years of their lives ticking them all off. The people I feel really sorry for are the ones who finished the lot on 277 in the 1980s and hung up their walking boots, only to find a load more of them added a few years later.

The reason for the disagreement is something called prominence, a measure of how high a mountain is in relation to the landscape around it. A free standing volcano in the middle of a low plain is more prominent than a higher peak surrounded by giants. There are various technical definitions for a mountain’s prominence, but perhaps the easiest one to understand is the amount of re-ascent a climber coming down from a higher mountain will have to do in order to get to its summit. For example, the south summit of Everest is 8749m high, considerably higher than the world’s 2nd highest mountain K2, which stands at only 8611m. But a climber coming down from Everest’s main summit to the col which divides the main and south summits, only has to re-ascend 10m to get back up to the south summit. The south summit therefore has a prominence of only 10m, and relative to Everest’s main summit is an insignificant pimple. If the climber then continues down to high camp at the South Col (7906m), they only have to climb back up 610m to reach the summit of Lhotse (8516m), the world’s 4th highest mountain. In terms of prominence, therefore, Lhotse is a bit of a minnow.

Isn’t this talk of prominence all a bit annoying? Actually I quite like it, as it makes me feel a bit more of a climber. In terms of literal height above sea level, I’ve only managed one of the world’s top 10, Manaslu in Nepal, coming it at no. 8. If we’re talking prominence, though, I can tick off Aconcagua in Argentina (no. 2), Kilimanjaro in Tanzania (no. 4), and Pico de Orizaba in Mexico (no. 7). And if we take the top 50 I’m doing even better. Apart from Manaslu I’ve only managed one more of the world’s literal highest, Muztag Ata in China, a lowly no. 43 on the list. But in terms of prominence I’ve climbed as many as six. As well as nos. 2, 4 and 7, I can count Ras Dashen in Ethiopia (no. 23), Mount Stanley in Uganda (no. 28), and Jebel Toubkal in Morocco (no. 36), and will be having a go at no. 46, Ritacuba Blanco in Colombia over Christmas.

This has been a post for trivia buffs, and if you’re interested in comparing the world’s highest mountains against the alternative list of the world’s most prominent, there are lists of both on that fount of all knowledge, Wikipedia.