1. Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 611756, China
2. Faculty of Engineering, Karary University, Khartoum 12304, Sudan
3. Institute for Geodesy and Geoinformation, University of Bonn, 53115 Bonn, Germany
4. College of Computer Science and Information Technology, Karary University, Omdurman 12304, Sudan