What is a mountain?

Max Kausch is a British/Argentinian mountaineer, and has climbed more mountains over 6000 metres in the Andes than anyone in history, clocking up 86 ascents so far.  We began climbing together several years ago, and realising that we might be able to climb ALL of the 6000ers in the Andes, began to ask the question 'what is a mountain?'

People have been making lists of mountains for many years - for example Munro made the famous list of mountains above 000 ft in Scotland, Corbett followed this up with a list of mountains between 2500 ft and 3000 ft.  Frequently, a definition known as prominence is used to define the independence of a mountain.  What we're really saying with this criteria is that if we imagine a huge mountain, with a huge boulder on the side of it, near the summit.  At what point is that boulder a separate mountain?  To determine this using the prominence method, we walk from the summit of the mountain to the summit of the boulder, following the highest path we can.  We measure the altitude difference between the lowest point on that path, and the peak of the boulder.  If that drop is above a certain value, we call that boulder an independent mountain, and that drop is called the 'prominence'.

The prominence was the measure used to determine the independence of mountains all over the world for many decades. Even in the UK, mountain lists are drawn up requiring prominences of 100 ft (Donalds), 150 metres (Marylins), 500 ft (Corbetts) and so on.  And these lists are only in the UK!  What about the rest of the world?

Well, the problem is that if you want to look at mountains in Wales, you set the prominence criteria to be pretty low (tens of metres), or else you end up with about three independent mountains.  It's a little better for the Alps, but still, you set a different number.  In the Andes, and in the Himalayas, that number has to be much higher - the mountains are so much higher in these regions that otherwise everything is an independent mountain.

  

Dominance

There is another way of measuring the independence of a mountain, and it's called dominance.  This is simply the prominence divided by the altitude of the mountain of interest, so it scales with the height of the mountain.  Now, for a mountain to be independent in the Himalayas, it has to drop much further to that key col, than would be the case in Wales.  We can set a single number, and apply it to all mountain ranges all over the globe!  This is much more satisfactory to me - although there is still an arbitrary threshold, that number is applied to all mountain ranges.

If our dominance is under 7% then our mountain is not independent, and if it's over 7% then it is!

We choose a value of 7% dominance in this study, but other values are of course possible, for example some people have chosen 3%.  This is usually done to create a list of mountains in a region that includes the widely acknowledged mountains of that area.  7% generally manages this, without including too many peaks.  As mentioned earlier, the value of this number is arbitrary, and this is the only arbitrary variable in this work.  Having decided this, Max and I needed to get to work to calculate an objective list of mountains in the Andes.  For this, we needed some data. 

Here's where the Earth Observation Science Group at the University of Leicester came in.  They're a research group specialising in monitoring the Earth's environment using satellite data.  They happened to have some data to hand that we could use, and agreed to help us.  The other piece of the puzzle that was necessary was access to a supercomputer to do the analysis - the father mountain could be on the other side of the data set from the mountain of interest, so where possible it's best not to chop the data into segments for analysis or you could miss the true key col, and the true father mountain.  The data set consists of a simple (but massive) grid of 20,000 by 80,000 data points, latitude, longitude and altitude.  

 

Right, now it's time to open the file, and find every point in that map where there is a local maximum over 6,000 metres and write some clever code to work out which ones of these are independent.  Next, set it all running on the supercomputer and wait....

Max and I waited anxiously for the results, and I ran the code for mountains over 6000 metres, and over 5000 metres.  Our 6000m list matched that already in existence, generated many years ago, although it also threw up some extra peaks that are sufficiently close to the 6000m borderline that they really ought to be climbed with a high precision GPS.  When I say close, I mean within 5 metres or so, and we know that the synthetic aperture radar that was used to take the data underestimates the altitude of the peaks (as the return signal comes from a broader area than just the peak of the mountain), so I actually believe there are likely to be 106 mountains above 6000 metres in total.  Max and I still have quite a few of these to go for!

The other thing we did was run the code on mountains above 5000 metres.  This meant going through many tens of thousands of potential mountains to ascertain which ones were independent, but when this list was complete, we realised something astonishing.  We had discovered dozens of mountains that didn't appear on any previous lists, many of which had no names, and we believed had never been climbed.  We set out immediately to climb as many of these as possible, with generous support for our initial expedition from the Mount Everest Foundation and the British Mountaineering Council.  You can find out more about our climbing adventures by reading my mountain blogs on this website - there are plenty of stories to be told!