http://dx.doi.org/10.1007/3-540-56869-7_2 Ralf Hartmut Güting Markus Schneider Realms: A Foundation for Spatial Data Types in Database Systems. 14-35 SSD/1993/P014.pdf 1993 conf/ssd/93 SSD db/conf/ssd/ssd93.html#GutingS93