Y Trechol – a dominance-based list

In my previous two blogs I said that I thought there weren’t any UK dominance-based mountain lists but it turns out that I was wrong!

Prominent (pardon the pun!) list compiler Myrddyn Phillips has been compiling a list of Welsh hills with a dominance of 50% or more and a prominence of at least 30 metres which he has called “Y Trechol” which is Welsh for “The Dominants”.

He has been publishing the list in parts on his “Mapping Mountains” blog since late 2015 and as I had added a comment to one of the instalments in early 2016, my initial statement about the absence of such lists now looks a little foolish.  After reading Myrddyn’s rationale behind the creation of his list it appears that it is likely that there will be hillwalkers who have probably toyed with the idea of dominance in relation to hill lists but perhaps haven’t published their thoughts.

Maybe dominance as a measure of mountains will become the vogue!

Prominence and a new mountain characteristic

Most modern mountain lists are defined by three characteristics –

• geographical boundary
• height
• prominence

I have talked about the Nuttalls in my previous two blogs so that list seems appropriate to use to demonstrate the point.  For a summit to be classed as a Nuttall, it must;

• be located in either England or Wales
• have its summit at or above 2000 feet
• have a prominence of at least 50 feet – later amended to 15 metres (49.21 feet)

Of the 3 criteria, the least known and understood is prominence.  So what is it?

Otherwise known as topographic prominence, Wikipedia defines it thus:

“prominence characterizes the height of a mountain or hill's summit by the vertical distance between it and the lowest contour line encircling it but containing no higher summit within it. It is a measure of the independence of a summit.”

In other words, if you were at a summit and wanted to travel to higher ground, what is the least possible descent you would have to make on that journey.  The amount of vertical descent is a summit’s prominence.

Prominence gives an indication of how far a summit is above its surroundings and becomes more obvious on steep sided hills and mountains; it is far less obvious when on plateaus or rolling terrain such as the Peak District or the Pennine hills.

Prominence-based lists use a fixed figure in their criteria; 15 metres for Nuttalls, 150 metres for Marilyns and 500 feet for Corbetts and Grahams.

But there is no proportionality considered when prominence is used.  For the Nuttalls, a 2000 foot summit with 15 metres of prominence has the same validity of classification as a 3000 foot summit with the same prominence, despite the lower summit being 50% more prominent if you compare prominence to height.  It could be argued that the 3000 foot summit is “less” prominent.

I’ve thought about this for a number of years and I don’t know whether I came to the subject independently or whether I had subliminally remembered an article I had previously read.  However it happened, a mountain chronicler called Eberhard Jurgalski had written (in 2001) about this much earlier than I had thought about it.  He had considered “proportional prominence” and called it “orometrical dominance” which is known more simply as “dominance”.  So despite the sensational title of this blog, dominance as a mountain characteristic isn’t that new – it’s just not used much!

Because dominance is a proportion of a summit’s height, it is a value that is written as a percentage.  The maximum value of dominance is 100% and this would apply to the highpoints of individual land masses, usually islands.

For instance, Scotland’s highpoint Ben Nevis has a height of 1345 metres and a prominence of 1345 metres.  Divide the height by the prominence and you get a result of 1, which expressed as a percentage is 100.

Other selected examples of dominance are…

Scotland’s second highest summit, Ben Macdui – dominance 72.57%

Scotland’s third highest summit, Braeriach – dominance 35.57%

Wales’ highpoint Snowdon (Yr Wyddfa) – dominance 95.76%

England’s highpoint Scafell Pike – dominance 93.25%

I am not aware of any mountain lists covering the UK (or anywhere else) that uses dominance as a criterion for selecting summits.  But now I’m going to create one!