These are the publications that appeared in 2001. The most recent publications are here.
Shepard, M.K., B.A. Campbell, M.H. Bulmer, T. Farr and L. Gaddis (2001), The roughness of natural terrain: A planetary and remote sensing perspective, J. Geophys. Res. Vol. 106 , No. E12 , p. 32,777 (December 2001), DOI: 10.1029/2000JE001429.
Abstract