1. a Center for Environmental Sciences , Saint Louis University , St. Louis, MO 63103
2. b Institute of Remote Sensing and GIS , Peking University , Beijing 100871, PR China
3. c Laboratoire des Sciences de l'Image , de l'Informatique et de la Télédétection , LSIIT (UMR7005) , 67400, Illkirch, France
4. d Institute of Geographic Sciences and Natural Resources Research , Beijing 100101, PR China