1. School of Geography Science, Nanjing Normal University, 1 Wenyuan Road; Nanjing 210023 China
2. Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, 1 Wenyuan Road; Nanjing 210023 China
3. State Key Laboratory of Resources and Environmental Information System; Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A Datun Road; Beijing 100101 China
4. Department of Geography; University of Wisconsin - Madison, 550 North Park Street; Madison Wisconsin 53706 USA
5. Institute of Geography, Physical Geography, University of Tübingen; Rümelinstraße 19-23 D-72074 Tübingen Germany