Surface Elements and GTOPO30

I’m writing code to calculate prominence, find key saddles, and graph divide trees. So I need elevation data. The U.S. Geological Survey has data for the entire planet: GTOPO30 It’s in digital elevation model (DEM) format.

The data points occur at every 30 arc seconds of latitude and longitude, which puts them about half a mile apart. The data is split into 33 files, and at 2 bytes per data point, the files are 50M when uncompressed. The 2 bytes contain the elevation in meters, or -9999 for ocean water.

The prominence article has a section on surface networks which gives a hint for how to proceed with DEM data. Dry land data points are classified as one of six surface elements based on the relative elevation of the four neighboring data points: summit, slope, ridge, channel, saddle, pit.

The prominence article doesn’t say how to classify data points with neighbors at the same elevation. At the resolution of the GTOPO30 data, this is common. I downloaded the file which contains Southern California, and I found that 21.26% of the dry land points were in this category.