David O'Sullivan Alasdair Turner Visibility graphs and landscape visibility analysis. 221-237 2001 15 International Journal of Geographical Information Science 3 http://taylorandfrancis.metapress.com/openurl.asp?genre=article&issn=1365-8816&volume=15&issue=3&spage=221 db/journals/gis/gis15.html#OSullivanT01