Implementation of the Riley et al (1999) Terrain Ruggedness Index
The algebraic approximation is considerably faster. However, because
inclusion of the center cell, the larger the scale the larger the divergence
of the minimum value.
Recommended ranges for classifying Topographic Ruggedness Index:
0-80 - level terrain surface.
81-116 - nearly level surface.
117-161 - slightly rugged surface.
162-239 - intermediately rugged surface.
240-497 - moderately rugged surface.
498-958 - highly rugged surface.
gt 959 - extremely rugged surface.