A spatial relation, specifies how some object is located in space in relation to some reference object. When the reference object is much bigger than the object to locate, the latter is often represented by a point. The reference object is often represented by a bounding box.
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In Anatomy it might be the case that a spatial relation is not fully applicable. Thus, the degree of applicability is defined which specifies from 0 till 100% how strongly a spatial relation holds. Often researchers concentrate on defining the applicability function for various spatial relations.
In spatial databases and Geospatial topology the spatial relations are used for spatial analysis and constraint specifications.
In cognitive development for walk and for catch objects, or for understand objects-behaviour; in robotic Natural Features Navigation; and many other areas, spatial relations plays a central role.
Commonly used types of spatial relations are: topological, directional and distance relations.
Topological relations
Main article DE-9IM.The DE-9IM model expresses important space relations which are invariant to rotation, translation and scaling transformations.
For any two spatial objects a and b, that can be points, lines and/or polygonal areas, there are 9 relations derived from DE-9IM:
Directional relations
Directional relations can again be differentiated into external directional relations and internal directional relations. An internal directional relation specifies where an object is located inside the reference object while an external relations specifies where the object is located outside of the reference objects.
Distance relations
Distance relations specify how far is the object away from the reference object.
Relations by class
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 objectss, 3 for volumetric objects. So, any object, in a 2D modeling, can by classifyed 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:
More complex modeling schemas can represent an object as a composition of simple sub-objects. Examples: represent in a astronomical map a star by a point and a binary star by two points; represent in geographical map a river with a line, for its source stream, and with an strip-area, for the rest of the river. These schemas can use the above classes, uniform composition classes (multi-point, multi-line and multi-area) and heterogeneous composition (points+lines as "object of dimension 1", points+lines+areas as "object of dimension 2").
Two internal components of a complex object can express (the above) binary relations between them, and ternary relations, using the whole object as a frame of reference. Some relations can be expressed by an abstract component, such the center of mass of the binary star, or a center line of the river.