What is Prominence and Why is it Important?

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.

Gunung Gede – not a Ribu or a Spesial

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.

Gunung Halimun – a Spesial

Another interesting example is Gunung Salak (2,211 m) and Gunung Halimun (1,913 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 922 m prominence – just 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.

Analysis of Prominence

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.

Data

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.

Methods

1. A preliminary screening for prominent peaks with a prominence of 1,000 meters was completed using the Winprom software version 2.3 (developed by Edward Earl) and SRTM 30 second data in “divide tree” format kindly provided by Edward;
2. The preliminary list of Ribus was imported into ESRI ArcGIS 9.3 as a point “shapefile” for further analysis in relation to the SRTM v4 data;
3. The SRTM v4 raster data were converted to 25 metre contours using ArcGIS Spatial Analyst extension;
4. The point shapefile representing the Ribus was overlaid on the SRTM v4 data to extract a more accurate maximum elevation value, which was added as an attribute to the point; and
5. Finally, the point Ribus shapefile was compared with the contours to determine what was the lowest contour that completely enclosed the point. For example, the the lowest contour surrounding Gunung Rinjani on Lombok island is the coastline at 0 metres elevation because Rinjani is the highest point on the island; therefore the prominence of Rinjani is the same as its elevation.

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, which in most parts of Indonesia can provide superior prominence values.to SRTM v4.

Some graphics and further explanation of data and methods will be provided in the near future

Interesting Ribu Facts

There are currently 193 Ribus, plus 49 Spesial peaks.

• Highest Ribu: Carstensz Pyramid, Puncak Jaya (4,884 metres elevation and prominence) Papua
• Lowest Ribu: Tombia (1,022 metres elevation) Sulawesi Tengah
• Most western Ribu: Hulumasen (2,310 metres elevation) Aceh
• Most eastern Ribu: Cycloop (2,000 metres elevation) Papua Timur
• Most northern Ribu: Seulawah Agam (1,810 metres elevation) Aceh
• Most southern Ribu: Wanggameti (1,225 metres elevation) Nusa Tenggara Timur
• Province with the most Ribus: Papua
• Province with the most Spesials: Nusa Tenggara Timur