1. a Department of Geography & Environmental Science, University of Zimbabwe, PO Box MP167, Mt Pleasant, Harare, Zimbabwe
2. b International Institute for Geo-Information Science and Earth Observation, Hengelosestraat 99, PO Box 6, NL-7500 AA Enschede, The Netherlands
3. c International Institute for Geo-Information Science and Earth Observation, Hengelosestraat 99, PO Box 6, NL-7500 AA Enschede, The Netherlands