1. Department of Geography; University of Wisconsin-Madison; Madison WI 53706 USA
2. School of Geographical Science; Nanjing Normal University; Nanjing Jiangsu 210023 China
3. Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application; Nanjing Jiangsu 210023 China
4. State Key Laboratory of Resources and Environmental Information System; Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences; Beijing 100101 China
5. Department of Geography and Institute for the Environment; University of North Carolina; Chapel Hill NC 27599 USA