[1] National Center for Geographic Information and Analysis, Department of Geography, State University of New York at Buffalo, Buffalo, NY 14261-0023, U.S.A. Email: dmark@geog.buffalo.edu.

[2] GIS Centre, Universitat de Girona, Pl. Ferrater Mora, 1, 17071 Girona, Spain. Email: comas@skywalker.udg.es.

[3] National Center for Geographic Information and Analysis, Department of Spatial Information Science and Engineering, and Department of Computer Science, University of Maine, Orono, ME 04469-5711, U.S.A. Email: max@mecan1.maine.edu.

[4] Department of Geography, University of Minnesota-Duluth, Duluth, Minnesota 55812, U.S.A. Email: sfreunds@d.umn.edu.

[5] Departamento de Geografía y Ordenación del Territorio, Universidad de Extremadura, Avda. Quijotes, s/n, E-10071 Cáceres, Spain. Email: ghhum02@sis.ucm.es.

[6] Departament de Geografia, Edifici B, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain. Email: ILGE5@cc.uab.es.