We would like to thank the referees for their recommendations and valuable suggestions. The work is supported by the Key Laboratory of Surveying and Mapping Technology on Island and Reef, National Administration of Surveying, Mapping and Geoinfomation (2013A03), National key scientific instrument and equipment development projects (2013YQ120343), the National Natural Science Foundation of China ( 11401422 ) and the National Natural Science Foundation of China ( 11371075 ).