Reference objects represented by a bounding box or another kind of "spatial envelope" that encloses its borders, can be denoted with the maximum number of dimensions of this envelope: 0 for punctual objects, 1 for linear objects, 2 for planar objects, 3 for volumetric objects. So, any object, in a 2D modeling, can by classified as "point", "line" or "area" according to its delimitation. Then, a "type of spatial relation" can be expressed by the class of the objects that participate in the relation: