1. Department of Geo-informatics, Central South University, Changsha, China
2. Department of Computer Science and Engineering, University of Minnesota, Minneapolis, MN, USA
3. Institute of Space and Earth Information Science, The Chinese University of Hong Kong, Shatin, Hong Kong, China
4. Center for Geospatial Information Science, University of Maryland, College Park, MD, USA
5. Department of Geographical Sciences, University of Maryland, College Park, MD, USA