Topographical Prominence Explained

07 Apr 2016 18:37 #67732 by AndrewP
During my 12 day GT with Ghaznavid in 2014/2015, he suggested to me the concept of prominence, and how it could be used to tell if a particular bump on a ridge should be classified as a peak or not.

Ghaznavid spoke about the highest of the low points and suggested that this can be 1000’s of kilometres away from the potential mountain we are interested in. It made no sense to me either.

When I got home, I searched on Google, and after a bit of reading, it made more sense. The Wikipedia article ( en.wikipedia.org/wiki/Topographic_prominence ) describes prominence really well. This is actually how most mountains around the world are classified, although amazingly the definition is not consistent. In the UK, Marilyn’s have a prominence of 150m although a cutoff of 15m is used for the “mountains of England and Wales”. In the 48 states of the USA, they use a cut off of 300 ft to list the 14000’er peaks and in Alaska 500 ft is used. In thus seems reasonable that we could use topographical prominence in the Drakensberg, and that we could chose a cutoff that makes sense according to local conditions.

The Wikipedia article above provides what is probably the easiest way to describe prominence. Imagine if we were to slowly raise the level of the sea until the peak we are interested in becomes the highest point on an island. At that point, the prominence is equal to the height of the peak on this imaginary island.

As an example, let consider the prominence of two peaks, Beacon Butress and the Sentinel. We first need to look at the map to work out the line of the watershed linking these peaks to a higher peak, which in this case is Mont-Aux-Sources. This is shown on the map below.



An approximate side profile view of this watershed is shown below.



Sentinel will obviously be measured off a higher, parent peak of Mont-Aux-Sources, and thus the key saddle is the lowest point along the watershed between the 2 peaks, this being 2919m at the saddle between Sentinel and Beacon Buttress. Sentinel’s prominence is thus 3072m – 2919m = 153m.

Beacon Buttress must also be measured against a higher point, and thus could be measured against either Sentinel or Mont-Aux-Sources. In this case, the key saddle is the higher of the 2 saddles, which is the saddle near the top of the chain ladder at 3015m. Beacon Buttress thus has a prominence of 3126m – 3015m = 111m, and its parent peak is also Mont-Aux-Sources.
Prominence works across borders really well. The watershed you follow to a higher peak can extend in any direction. So, for example the parent peak for Mafadi is Makheka, which is on the same ridge line but about 8km into Lesotho.

Makheka which is the 2nd highest peak in Lesotho of course gets its prominence off Thabana Ntlenyana, which in turn gets it prominence off Mt Kilimanjaro which is 1000’s km away.

One of the great things about prominence is that we measure against any point that is higher than us, even if that point is not in itself a peak. This means for example that a ridge that steadily gains height with a few bumps on it, tends to cancel itself out bit by bit, even if the top of the ridge is far away from an much higher than the bottom part of the ridge. We only keep the significant bumps with a drop off on both sides.

Further, it provides a fair way to classify if a freestanding pinnacle should be included or not, but using the same criteria as for the peaks that form part of a ridge.

The heights of saddles are not normally marked accurately on a map, so a whole lot of fieldwork is needed if we want to accurately determine the prominence of each berg peak. As a GPS is only so accurate, it is best to tag both the saddle and summit with as little time lag between as possible. Ideally, a GPS with a built-in barometric altimeter will be used, but even this will only be so good.

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The following user(s) said Thank You: intrepid, DeonS, ghaznavid, tonymarshall, supertramp, Redshift3

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