1 GITECO Research Group, Universidad de Cantabria, Av. de los Castros 44, 39005 Santander, Spain. Electronic address: jatod@unican.es.
2 Department of Geographical Engineering and Techniques of Graphic Expression, Universidad de Cantabria, Av. de los Castros 44, 39005 Santander, Spain.
3 GITECO Research Group, Universidad de Cantabria, Av. de los Castros 44, 39005 Santander, Spain.