The following restrictions apply to every pair of sets. A major drawback of this approach is the failure to describe true one-dimensional objects. This was the motivation for the proposal by Clementini et al. They extended Egenhofer's approach by introducing points and lines as additional object types and the dimension of intersections as new feature for discriminating more cases. Three types of geometric objects are modeled.
Regions have to be convex, connected and without holes.
Lines and arrows must not be self-intersecting, are either circular or directed, and have exactly two end points. Points are elements of lines and describe their start or end points. The boundary of a region is a circular line. The interior of an object is the object without its boundary. Both approaches cannot deal with concave objects.