1. a State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing , Wuhan University , Wuhan , Hubei , 430079 , PR China
2. b School of Info-Physics and Geomatics Engineering , Central South University , Changsha , Hunan , 410083 , PR China
3. c Department of Geography , East Carolina University , Greenville , NC , 27858 , USA