Terrain Ruggedness Index

The Terrain Ruggedness Index is originally devised by

Riley, Shawn J., Stephen D. DeGloria, and Robert Elliot. “A Terrain Ruggedness Index That Quantifies Topographic Heterogeneity.” Intermountain Journal of Sciences, 5: 1–4 (1999), 23–27.

to quantify topographic heterogeneity in wildlife habitats that provide hiding for prey and lookout posts.

Nunn and Puga (2012) use this index to measure "small-scale terrain irregularities, such as caverns, caves, and cliff walls, that afforded protection to those being raided during the slave trades" (p. 21). They find that its country-level average is positively correlated with GDP per capita today in Africa.

On his webpage, Diego Puga makes public this index at the level of individual cells on a 30 arc-seconds grid across the surface of the Earth.