1. a Spatial Information Research Centre, Department of Information Science , University of Otago , PO Box 56, Dunedin, New Zealand E-mail:
2. b Discovery Software , 36 Elm Park, Paignton, Devon, TQ3 3QQ, UK
3. c Uphill, Lustleigh , Newton Abbot, Devon, TQ13 9SE, UK
4. d Department of Geographical Sciences , University of Plymouth , Drake Circus, Plymouth, Devon, PL4 8AA, UK