1. Smart City Research Center, School of Automation, Hangzhou Dianzi University, Hangzhou 310012, China
2. Department of Geography, University of Wisconsin–Madison, Madison, WI 53706, USA
3. State Key Laboratory of Resources and Environmental Information System, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China
4. Key Laboratory of Virtual Geographic Environment, Ministry of Education, Nanjing Normal University, Nanjing 210023, China