Well, it depends on where the sun is, and how directly you want the pixels to be facing the sun. If the sun is due east (aspect = 90), and you want to include pixels that are facing within 20 degrees of directly toward the sun, then you would use aspects between 70 and 110. Does that make sense? - Peg

Thanks Peg, Got the idea. Could you please point towards detailed references related to Topographic Measures beyond the ENVI help? Cheers, Simit

The only one that I know about is the reference cited in the ENVI Help system under Using ENVI > Topographic Tools > Using Topographic Modeling, which is: Wood, Joseph The Geomorphological Characterization of Digital Elevation Models, Ph. D. Thesis, University of Leicester, Department of Geography, Leicester, UK, 1996. - Peg