http://dx.doi.org/10.1007/3-540-60159-7_18 Mohammad Nabil John Shepherd Anne H. H. Ngu 2D Projection Interval Relationships: A Symbolic Representation of Spatial Relationships. 292-309 SSD/1995/P292.pdf 1995 conf/ssd/95 SSD db/conf/ssd/ssd95.html#NabilSN95