Department of GIS, Lanzhou Jiaotong University, Lanzhou 730070, China;
Department of Geography & Environmental Management, University of Waterloo, Waterloo, Ontario N2L 3G1,Canada;
Gansu Provincial Engineering Laboratory for National Geographic State Monitoring, Lanzhou 730070, China;
Department of GIS, Lanzhou Jiaotong University, Lanzhou 730070, China;
Department of Geography & Environmental Management, University of Waterloo, Waterloo, Ontario N2L 3G1,Canada;
Department of Geography & Environmental Management, University of Waterloo, Waterloo, Ontario N2L 3G1,Canada;