1. a Department of Geography , Michigan State University , East Lansing, Michigan 48824–1115 arbogas2@pilot.msu.edu
2. b U.S. Army Topographic Engineering Center , Directorate of Operations Terrain Analysis Center , Alexandria, Virginia 22315–3864
3. c U.S. Military Academy , West Point, New York 10996
4. d Department of Geography , Michigan State University , East Lansing, Michigan 48824–1115