Joint Research Centre of the European Commission, Global Environment Monitoring Unit, Via Fermi 2749, TP 440, I-21027 Ispra (VA), Italy;Computer Science and NCRG, Aston University, Birmingham B4 7ET, UK;Department of Mineral Resources Engineering, Technical University of Crete, Chania 73100, Greece;Institute for geoinformatics (ifgi), University of Münster, Germany;Institute of Statistics, University of Klagenfurt, 9020 Klagenfurt, Austria