1. a Department of Geography , NERC Unit for Thematic Information Systems, University of Reading , Reading, RG6 2AB, England
2. b Logica Space and Defence Systems , Cobham Park, Downside Road, Cobham, Surrey, TK11 3LX, England
3. c Laboratory of Applied Soil Science, Aristotelian University of Thessaloniki , Thessaloniki, GR, 540 06, Greece