1. a Faculty of Earth Sciences , Utrecht University , 3508 TA, Utrecht, the Netherlands
2. b German Remote Sensing Data Center (DFD) , German Aerospace Center (DLR) , Oberpfaffenhofen, D-82234 Wessling, Germany
3. c Institute of Photogrammetry and Remote Sensing , Vienna University of Technology , Gusshausstraβe 27–29, 1040 Vienna, Austria