San Jacinto Peak and Mt. Baldy are often thought of as the second and third tallest mountains in Southern California, behind San Gorgonio. In fact, they rank #6 and #22. However, as the tallest mountains in their ranges, San Jacinto and Mt. Baldy are better known than the actual #2 and #3 summits: Jepson Peak (11,205) and Bighorn Mountain (10,997). They also have greater topographic isolation and topographic prominence than Jepson and Bighorn.
What are topographic isolation and topographic prominence, and why do they matter?
A peak’s topographical isolation is the air mileage to the nearest taller peak (the “parent.”) Bighorn and Jepson are less than a mile from San Gorgonio’s summit, giving them minimal topographic isolation. (San Gorgonio has a topographic isolation of 162 miles; its nearest taller neighbor is Nevada’s Mt. Charleston). San Jacinto is just over 20 miles from its parent, Bighorn and thus is an example of how a mountain can have a greater topographic isolation than its nearest taller. Similarly, while San Jacinto is taller than Mt. Baldy, Baldy’s isolation is more than double, at 42 miles.
Topographic prominence measures how much a mountain stands out from its surroundings. If you were to look at a topographic map of the mountain, the prominence would be the height distance between the summit and the lowest ring that completely circles the peak.
In the example below, “Cave Hill” (center) has an elevation of 570 and the lowest ring completely circling it is 530, meaning its prominence is 40. “Adams Hill” (lower left) has lower elevation (430) but greater prominence: the lowest ring completely circling it is 310, resulting in a prominence of 120 (430-310).
Topographic prominence is similar to topographic isolation in some ways but different in others. For example, San Jacinto has a higher prominence than San Gorgonio (8,339 feet vs. 8,294). These two peaks are respectively the sixth and seventh most prominent peaks in the continental U.S. – which probably comes as no surprise to anyone who has seen them first hand towering above the desert.
Types of prominence
“Clean” and “optimistic” (or “dirty”) are two different ways to measure a peak’s prominence. Clean is more conservative, assuming that the height of the key col (the highest of any number of low points between the peak and its nearest neighbor) is represented by the highest possible contour line on a map and the summit altitude (if not known exactly) is the lowest possible. Optimistic prominence is measured the opposite way: by assuming the lowest possible measurement for the key col and the for highest summit. (Some opt to use “interpolated” or “average” prominence – measured by splitting the difference between the other two techniques). The difference between clean and optimistic is typically twice the length of the contour line on the map.
This example from Peakbagger assumes a map with 40 foot intervals, a summit located between the 8,000 and 8,040 foot lines and a key col located between the 6,960 and 7,000 foot lines. The optimistic prominence would be 1,080 feet (8,040-6,960); the interpolated or average prominence would be 1,040 feet (8,020-6,980) and the clean prominence would be 1,000 feet (8,000-7,000). In this example, the difference between clean and optimistic prominence is 80 feet (1,080 vs 1,000) – double the length of the 40 foot contour interval.
A hiker’s “P-Index” refers to the number of peaks they have bagged with a prominence (in meters) greater than that same number. For example, if you have climbed 10 peaks with a prominence of more than 10 meters, your P-Index is 10. If you have climbed 50 peaks with a prominence of 50 meters or greater, your P-index is 50. It is a way to roughly measure both the quantity and quality of one’s peak-bagging exploits. (In case you were wondering, the highest possible P-index is approximately 1,500 – the number of peaks on Earth with a prominence of 1,500 meters or more.)
Again, why does this matter?
Do you have to know a mountain’s topographic isolation or prominence to enjoy it? Does a mountain have to have a high isolation or prominence to yield great views? Of course not. However, understanding these concepts can help predict what kind of views can be expected from a summit.
Here are the height, prominence and isolation measurements for several notable California peaks.