Acknowledgements
The authors are very grateful for assistance and comments provided by Prof. Xingfa Gu of Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences. This study was partially supported by the Fundamental Research Funds for the Central Universities (2013YB42), the National Natural Science Foundation of China (41301353, 41371361), the International S&T Cooperation Program of China (2012DFG21710), and the Science and Technology Subsidies to Leading Scientists by National Administration of Surveying,Mapping and Geoinformation. The authors were grateful for the comments and suggestions of the two anonymous referees and the editor, which had leading to improvements in the presentation of this article.