http://doi.acm.org/10.1145/181550.181566 db/journals/sigmod/MedeirosP94.html Claudia Bauzer Medeiros Fatima Pires 107-115 sigmodR/23-1/P107.pdf 1994 23 SIGMOD Record 1 db/journals/sigmod/sigmod23.html#MedeirosP94 http://www.acm.org/sigmod/record/issues/9403/GIS.ps