Jildou Louwsma Sisi Zlatanova Ron van Lammeren Peter van Oosterom Specifying and Implementing Constraints in GIS - with Examples from a Geo-Virtual Reality System. 531-550 2006 10 GeoInformatica 4 http://dx.doi.org/10.1007/s10707-006-0345-5 db/journals/geoinformatica/geoinformatica10.html#LouwsmaZLO06