1. College of Geodesy and Geomatics, Shandong Univ. of Science and Technology, Qingdao 266590, China.
2. College of Geodesy and Geomatics, Shandong Univ. of Science and Technology, Qingdao 266590, China (corresponding author).
3. State Key Laboratory of Resources and Environmental Information System, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China.