1. Institute of Geodesy and Photogrammetry , Department of Civil, Environmental and Geomatic Engineering, ETH Zurich, Robert-Gnehm-Weg 15, 8093 Zurich , Switzerland
2. Department of Applied Mathematics, University of Alicante , 03690 San Vicente del Raspeig Alicante , Spain
3. Seminar for Applied Mathematics, Department of Mathematics , and ETH AI Center, ETH Zurich, Rämistrasse 101, 8092 Zurich , Switzerland