1. Department of Geographic Information Science; Nanjing University; Nanjing Jiangsu Province 210023 China
2. Jiangsu Provincial Key Laboratory of Geographic Information Science and Technology; Nanjing University; Nanjing Jiangsu Province 210023 China
3. Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application; Nanjing Jiangsu Province 210023 China
4. Department of Geography; University of Wisconsin-Madison; Madison WI 53706 USA