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In topography, prominence, also known as autonomous height, relative height, shoulder drop or prime factor (in Europe), is a concept used in the categorization of hills and mountains. It describes how tall a peak is relative to neighbouring peaks, and in a way that makes precise the intuition that the world's second-tallest mountain is in fact K2 (height 8,611 m, prominence 3811 m), and not, say, Everest's South Summit (height 8749 m, prominence about 10 m).
Definition of prominence
There are several equivalent definitions:
- The prominence of a peak is the height of the peak’s summit above the lowest contour line encircling it and no higher summit (the base contour of the peak).
- For a peak with prominence P metres, to get from the summit to any higher terrain, one must descend at least P metres, whatever route is taken.
- For all peaks except the highest on a landmass, prominence is the vertical difference between the peak’s summit and the highest "col" connecting it to a higher area of land. The usual meaning of col is any low point on a ridge. However, in this definition a specialised, non-standard meaning is given to "col", namely, the lowest point on a ridge connecting a summit to a higher area of land. To determine the "highest" requires investigating all possible ridge routes.
The highest connecting "col" is called the key col, linking col or just link.
A more detailed explanation is given below.
The parent of the peak is on this higher terrain but if there are several higher peaks there are various ways of defining which one is the parent. These concepts give ways of putting all peaks on a landmass into a hierarchy showing which peaks are subpeaks of which others. For example, in the diagram on the right, the middle peak is a subpeak of the right peak, which is in turn a subpeak of the left peak, which is the highest point on its landmass. The key col and prominence are marked for each subpeak.
Prominence in mountaineering
Prominence is interesting to mountaineers because it is an objective measurement that is strongly correlated with the subjective significance of a summit. Peaks with low prominences are really just subsidiary tops of some higher summit. Peaks with high prominences tend to be the highest points around and are likely to have extraordinary views. In the U.S., 2000 feet of prominence has become an informal threshold that signifies that a peak has major stature.
Many lists of mountains take topographic prominence as a criterion for inclusion. John and Anne Nuttall's The Mountains of England and Wales uses 15 m (about 50 ft), whereas Alan Dawson's list of Marilyns uses 150 m (about 500 ft). Lists with a high topographic prominence inclusion criterion tend to favour isolated peaks or those that are the highest point of their massif; a low value, such as the Nuttalls', results in a list with many summits which may be viewed by some as insignificant.
Interesting prominence situations
The key col and parent peak are often close to the subpeak but this is no always the case, especially for major peaks. It is only with the advent of computer programs and geographical databases that thorough analysis has become possible.
- The key col of Mount McKinley in Alaska (6194 m) is a 37 m col near Lake Nicaragua (or was before the Panama Canal was cut!). McKinley’s parent peak is Aconcagua, Argentina (6960 m) and its prominence is 6157 m. Put another way – if sea level rose 37 m North and South America would be separate continents and McKinley would be 6157 m above sea level.
- Mount Whitney (4418 m) has its key col 1022 km (635 miles) away in New Mexico at 1347 m. Its parent peak is Pico de Orizaba (5611 m), the highest mountain in Mexico. Orizaba’s key col is in British Columbia.
More detailed explanation
The definition of prominence "the vertical difference between the peak’s summit and the highest col connecting it to a higher area of land" deserves more explanation.
- "summit". The highest point on a peak or subpeak. The work "peak" is here being used generally to mean any mountain or hill.
- "col". A col is a low point on a ridge. There may be several cols on a particular ridge between two peaks – one of the cols will be the lowest. This lowest col will often be the one a mountain pass crosses as it goes over the ridge. Just in the particular context of this definition of prominence, the word "col" is being used to mean "lowest col on a ridge".
- "higher terrain". Which higher terrain is meant? In fact to begin with it could be any higher area of land: the actual area is still unknown. It is likely to be nearby but could be very remote. One needs to consider all higher areas in order to determine which one, in retrospect, turns out to be the one in question (which is also the one containing the parent peak).
- "the highest col". In the diagram above, the key col of the right hand peak is shown as the lower of the two cols. What has gone wrong? One must consider every possible ridge route from the peak to every higher land area. Each ridge will have a lowest col. There may be several ridge routes between a peak and a single area of higher terrain. All these lowest cols are listed, one for each ridge. Finally, the highest of all these lowest cols is identified. This, then, is the key col. However, for any peak, analysis only extends over the continent or island the peak is on.
To calculate the key col and parent peak seems like a good job for a computer and fortunately Edward Earl thought so too. His program WinProm can be used to make the very involved calculations required, based on the USGS Digital Elevation Model database. The underlying mathematical theory is called "Surface Network Modelling ".
Sometimes a definition is given such as "the lowest col connecting the peak to a higher summit". This is very misleading. It is true that the key col is the lowest col on the ridge. However, the ridge has been selected such that its lowest col is the highest one possible considering all ridge routes.
In the light of this complication, to visualise the situation it may be easier to use the first definition in this article "the height of the summit above the lowest contour line encircling it and no higher summit" Call the summit "Home Peak" and imagine sea level raised so that the top of Home Peak is a tiny island. There are other islands around, all with higher summits, the "Original Isles". Imagine the sea level dropping. The islands all get bigger and new ones (with lower summits) appear. Islands start merging together. They may take on contorted outlines of winding ridges if the topography is complex. A map of Sulawesi is a nice example. As sea level drops further a critical, unique event occurs. The island of Home Peak just becomes connected with one of the Original Isles. The point of connection (isthmus) is the key col.
Since mountain altitudes are measured above sea level, the analysis above only extends over the geographical island or continent being studied.
There are different systems for defining the parent peak, which can lead to different results. One is that it is the highest point on the new, merged island. Another definition is that, following the ridge route from the peak through its key col and onwards, the parent is the first peak higher than the starting point.
- http://www.peaklist.org a website about mountain prominence
- Prominence and Orometry a detailed and lucid account by Aaron Maizlish of the theory of prominence
- Mountain Hierarchies a description of the different systems of defining parent peak
- Surface Network Modelling on the Center for Advanced Surface Analysis website
- Surface Network Modelling a paper by Sanjay Rana and Jeremy Morley
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