Most hillwalking lists have tried to use criteria to distinguish significant independent peaks from minor or subsidiary summits. These criteria are not always consistent and precise. Topographic prominence is a clear way of defining the independence of a summit.
There are several equivalent ways of defining prominence. One is to define the prominence of a peak as its height above the lowest contour that surrounds that peak and no higher summit.
Letís imagine an island with 3 peaks:
- Peak A with summit at 840 metres above sea level
- Peak B with summit at 550 metres above sea level
- Peak C with summit at 660 metres above sea level.
Since Peak A is the highest on the island the lowest contour enclosing it is the shoreline of the island. Thus the prominence of A is equal to its elevation.
For Peak C the lowest contour that encircles it and no higher summit is the contour at the lowest point between Peak C and Peak A. This lowest point is known as the key col.
The prominence of Peak C is its elevation above its key col.
The key col has an elevation of 400 m so Cís prominence is (660-400) m = 260 m.
An equivalent way to visualise prominence is the flooded island analogy.
To complete the account on defining prominence it is worth mentioning the notion of proportional prominence or orometrical dominance. The dominance D of a peak is defined as:
D = P/H ◊ 100%
where P is the prominence of a peak and H is the summit height.
By definition the highest point in an island e.g. Snaefell on the Isle of Man is 100%.