RT Journal Article SR Electronic T1 Landform equations fitted to contour maps JF American Journal of Science JO Am J Sci FD American Journal of Science SP 616 OP 627 DO 10.2475/ajs.263.7.616 VO 263 IS 7 A1 Troeh, F. R. YR 1965 UL http://www.ajsonline.org/content/263/7/616.abstract AB