1-2-5 Prominence Categories

It has often been observed that mountain ranges look very similar regardless of their scale. Mountains in Scotland may have a similar shape to those in Norway but have half the height. Swiss alpine peaks may have similar shapes to ones in the Karakoram but, while the Swiss peaks may be over 4000 metres, those in the Karakoram may be over 7000 metres.

Given this, it makes sense to classify peaks into groups separated by roughly equal geometric leaps. However, it makes sense to ensure that the groups are bounded by round numbers in metres. Jim Bloomer and Roddy Urquhart have therefore devised a system of prominence categories or bands, where the groups are separated alternately by multiples of 2 or 2.5, for example 10 m, 20 m, 50 m, 100 m, etc. The principle of using these multiples is referred to by the authors as the 1-2-5 Principle.

The authors use the following notation to describe prominence categories based on the 1-2-5 Principle:

P5000 is the top prominence category and there are only 6 P5000s across the globe, with just Everest in Eurasia, Kilimanjaro in Africa and 2 each in N & S America.

There are 473 P2000 peaks spread over all continents and various islands.

A further 7 peaks have less than 5000 m prominence but are still more prominent than the second most prominent Himalayan peak - Nanga Parbat with prominence of 4608 m/height 8125m

In the European Alps, there are 13 P2000s and 170 P1000s. In the UK there are just 3 P1000s but 155 P500s, 577 P200s and 829 P100s. The Lake District in England has 4 P500s, 19 P200s, 41 P100s and 50 P50s.

The 1-2-5 principle can scale to smaller undulations too. Suppose somebody in the Netherlands were interested in classifying their hills. Those hills with a height above 100 metres might be interesting; and definitely those with a prominence of 100 metres given that the Netherlands is by definition a “low country”. In a very flat area, such as the Western Netherlands or Norfolk, even the smallest P10 and P20 categories could potentially be of interest.

[Photo courtesy of NASA]

Thus the 1-2-5 principle can be used to categorise small undulations to the largest mountains. There is also no reason why it should not be applied to the topography of other planets and moons. Thus the largest known mountain in the Solar System, Olympus Mons on Mars, which rises 27 km above the mean surface level of Mars, is a P20000 peak!