Boaz Ben-Moshe Paz Carmi Matthew J. Katz Approximating the Visible Region of a Point on a Terrain. 21-36 2008 12 GeoInformatica 1 http://dx.doi.org/10.1007/s10707-006-0017-5 db/journals/geoinformatica/geoinformatica12.html#Ben-MosheCK08