Key saddle

In topography, 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. A peak's key col is a unique point on this contour line and the parent peak is some higher mountain, selected according to various objective criteria.

By convention, the prominence of Mount Everest, the Earth's highest mountain, is taken to equal the elevation of its summit above sea level. Apart from this special case, there are several equivalent definitions:

The following mental exercise may illustrate the meaning of topographic prominence. Imagine you are standing at the top of a peak and imagine that an imaginary sea level (based on an elevation above the Geoid) rises to your feet. Now slowly lower the imaginary sea level and an imaginary island appears beneath your feet. Your island will grow and will merge with other islands that emerge. Eventually, your island will touch an island with a higher peak than your island (i.e., an imaginary island that existed before you started lowering the imaginary sea level.) The summit of that island is the parent peak of your summit, the point at which the two islands touch is the key col of your summit, and the elevation rise from the key col to your summit is the topographic prominence of your summit.

The parent peak may be either close or far from the subject peak. The summit of Mount Everest is the parent peak of Aconcagua at a distance of 17,755 km (11,032 miles), as well as the parent of the South Summit of Mount Everest at a distance of 360 m (1200 feet). The key col may also be close or far from the subject peak. The key col for Aconcagua is the Bering Strait at a distance of 13,655 km (8,485 miles).[why? why not the nearest sea-level point?] The key col for the South Summit of Mount Everest is about 100 m (330 feet) distant.

Prominence is interesting to many mountaineers because it is an objective measurement that is strongly correlated with the subjective significance of a summit. Peaks with low prominence are either subsidiary tops of some higher summit or relatively insignificant independent summits. Peaks with high prominence tend to be the highest points around and are likely to have extraordinary views.

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