1. a Research Centre of the Government Geographic Information System, Chinese Academy of Surveying and Mapping , Lianhuachi West Road, Haidian District , Beijing , 100830 , PR China
2. b Geoinformation Science and Engineering College, Shandong University of Science and Technology , Qingdao , 266510 , PR China
3. c State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University , Wuhan , 430079 , PR China