1. a Department of Information Engineering and Computer Science , University of Trento Via Sommarive , 14 I-38123, Povo, Trento, Italy
2. b Faculty of Electrical and Computer Engineering, University of Iceland, Hjardarhaga 2-6 , 101, Reykjavik, Iceland
3. c Institute of Geodesy and Geoinformation, Department of Photogrammetry, University of Bonn, Nussallee 15 , 53115, Bonn, Germany