1. School of Public Affairs and Administration, University of Electronic Science and Technology of China, Chengdu 611731, China
2. School of Automation, University of Electronic Science and Technology of China, Chengdu 610054, China
3. College of Resource and Environment Engineering, Guizhou University, Guiyang 550025, China
4. School of Geographic Science, Southwest University, Chongqing 400715, China
5. Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803, USA