Bowon Kim Kyhyun Um 2D+ String: A Spatial Metadata to Reason Topological and Directional Relationships. 112-121 DiSC2_1/out/papers/ssdbm/00460112.pdf 1999 SSDBM http://computer.org/conferen/proceed/ssdbm/0046/00460112abs.htm db/conf/ssdbm/ssdbm99.html#KimU99