1. a Department of Land Surveying and Geo-Informatics , The Hong Kong Polytechnic University , Kowloon, Hong Kong E-mail:
2. b Lanzhou Jiaotong University , Lanzhou, 730070, China
3. c School of Resource and Environment Sciences , Wuhan University , China
4. d National Geometrics Center of China , 1 Baishengcun, Zizhuyuan, Beijing 100044, China