Mordechai Haklay Carolina Tobón Usability evaluation and PPGIS: towards a user-centred design approach. 577-592 2003 17 International Journal of Geographical Information Science 6 http://taylorandfrancis.metapress.com/openurl.asp?genre=article&issn=1365-8816&volume=17&issue=6&spage=577 db/journals/gis/gis17.html#HaklayT03