http://doi.acm.org/10.1145/122058.122065 db/journals/sigmod/GuntherB90.html Oliver Günther Alejandro P. Buchmann Research Issues in Spatial Databases. 61-68 sigmodR/19-4/P061.pdf 1990 19 SIGMOD Record 4 db/journals/sigmod/sigmod19.html#GuntherB90