# Interpreting Ruggedness Index calculated from SRTM Tiles

I am looking for help interpreting the values from Ruggedness Index calculations.

I downloaded SRTM tiles using the SRTM Downloader plugin in QGIS. Then I calculated ruggedness from SRTM tiles in QGIS with the Ruggedness Index function from the raster terrain analysis section of the toolbox, which "calculates the quantitative measurement of terrain heterogeneity described by Riley et al. (1999)." Then I sampled the ruggedness values from a list of points in Southern Arizona.

Of all the points, the highest ruggedness value I got was about 62. The lowest I got was zero.

These values are surprising to me and seem very low when I use this scale below, found here and here, to interpret these results. (Southern Arizona can be a very mountainous and rugged area.)

• 0-80m is considered to represent a level terrain surface
• 81-116m represents a nearly level surface
• 117-161m represents a slightly rugged surface
• 162-239m represents an intermediately rugged surface
• 240-497m represents a moderately rugged surface
• 498-958m represents a highly rugged surface
• 959-4367m represents an extremely rugged surface

Someone helping me said the issue could be that these categories are based on a DEM with 1km raster cells so the categories don’t fit well for the DEM i'm using. "In other words, a change of 60m in elevation in 1km distance is not equivalent to 60m in say, a 30m distance."

How do I figure out the raster cell size of the SRTM tiles I am using? And how can I interpret the ruggedness values on a categorical scale appropriate for my cell size?

I've tried to find the pixel size of my tiles in the following ways:

The unit of my ruggedness geotiff raster and SRTM tiles is degrees. The pixel size is 0.000277777777786595648,-0.0002777777777932086307, which i think is around 10,-10 meters. Perhaps the problem I'm having is that my ruggedness is calculated in degrees and I need to convert it to meters? Does this seem likely?

I'm not sure if the SRTM Downloader plugin gives me tiles with spatial resolution of 1 arc-second for global coverage (~30 meters) or 3 arc-seconds for global coverage (~90 meters). Can I figure this out by looking at the properties of my SRTM tiles? And once I figure this out, how should I categorize my ruggedness values?

Here are the properties of one of my SRTM tiles:

Here are the properties of my ruggedness geotiff file, which is the result of merging SRTM tiles: