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.