1. College of Information Science & Technology, Beijing University of Chemical Technology, Beijing 100029, P. R. China
2. Engineering Research Center of Intelligent PSE, Ministry of Education of China, Beijing 100029, P. R. China
3. Center for SDI and Land Administration, Department of Infrastructure Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia
4. State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing (LIESMARS), Wuhan University, Wuhan, Hubei 430079, P. R. China