It is often assumed that elevation (i.e height above sea level) is the most important criteria for identifying a mountain that is worth climbing – the thinking behind this idea is that “a big mountain is a good mountain”. Most of the time, this thinking does turn out to be correct. However, an alternative and increasingly popular approach is to assess the extent to which a mountain qualifies as a separate peak from its neighbouring mountains – its prominence. Similar to a number of hill and mountains lists worldwide, Gunung Bagging uses topographic prominence as the objective criteria with which to identify separate mountain or volcano peaks in Indonesia.
The following example will hopefully illustrate the difference between elevation and prominence. People may wonder why Gunung Gede does not feature in the list of Ribus. Gunung Gede is 2,958 metres above sea level (elevation) and lots of people climb it. However, nearby Gunung Pangrango is 3,019 metres in elevation. The lowest point on the ridge connecting the two peaks (also known as col, pass, or saddle) is approximately 2,520 metres. Gede elevation (2,958) minus col (2,520) equals roughly 438 metres prominence, which means that Gede is not a Ribu.
Gunung Gede 2,550 m contour highlighted in blue, which does not encircle any point higher than the summit of Gede.
Gede 2,525 m contour highlighted in blue, which encircles the higher summit of Gunung Pangrango.
The nearest higher summit to Pangrango is Ciremai at 3,078m (also in West Java) but the col between the two peaks is easily sufficient for both peaks to qualify as a Ribu by having over 1,000 m prominence. So, nearby Pangrango is a Ribu because it has more than 1,000m prominence. For the purposes of our list, Gunung Gede is seen as an interesting ‘part’ of Pangrango mountain rather than being a separate Ribu itself. Because Gede is a very interesting and popular peak, in certain circumstances it might be eligible for entry on the Spesial list for peaks which do not have 1,000m prominence. However, we regard Gede and Pangrango as very close neighbours and we would expect that anyone climbing Pangrango would visit Gede anyway as part of the same hike.
Another interesting example is Gunung Salak (2,211 m) and Gunung Halimun (1,929 m). There is a narrow ridge that “connects” Halimun to the higher Salak, which results in a col or saddle of Halimun being 991 m. Based on the difference between the elevation of Halimun and the col, Halimun has 938 m prominence – 62 metres below what is required to be a Ribu.
Gunung Halimun 1,000 m contour highlighted in blue, which does not encircle any point higher than the summit of Halimun.
|Gunung Halimun 975 m contour highlighted in blue, which encircles the higher summit of Gunung Salak.|
Since it is many kilometres from Gunung Salak, Gunung Halimun feels very much like a separate mountain; you certainly could not climb Salak and Halimun in the same trek as in the previous example of Pangrango and Gede. Therefore, we decided to include Halimun in the Spesial list of exceptional peaks because it is deemed to be of significant interest as an individual mountain area, and it comes very close to having the 1,000 metres prominence required to be a Ribu.
After we decided that 1,000 metres was the most suitable prominence to identify important mountain peaks across the Indonesian archipelago, the work began to actually create a comprehensive list and accurate geographic locations. Of course, any Indonesian island with a highpoint of 1,000 metres or more qualifies as a Ribu, so that was straightforward. Also, all the Indonesian Ultras (peaks in the world with a prominence of 1,500 metres or above) qualify as a Ribu. Examination of various maps enabled us to identify many more mountains that obviously had a drop all-round of well over 1,000m, but we wanted to follow a systematic approach using standard data and repeatable methods.
Elevation data used was the global shuttle radar topography mission (SRTM) v4, which is available for research purposes from Consortium for Spatial Information (CGIAR-CSI). The newer ASTER GDEM data are still considered to suffer from too many errors in areas of persistent cloud cover like Indonesia, so the radar-based STRM was considered the most reliable choice.
There are some potential errors in the prominence and elevation figures for the Ribus as a result of the input data and the method of analysis. The SRTM v4 has a published quality assessment (pdf 2.5 mb). The method of analysis described accurately identifies the summit elevation based in the SRTM v4, but the use of 50 metre contours means that the prominence values have some uncertainty. In the future, we intend to consult topographic maps produced by the Indonesian mapping agency (Bakosurtanal), which in some parts of Indonesia can provide superior prominence values to SRTM v4.