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2.2 Classification and Intended Use of Objects

This standard defines three classes of spatial objects. Two classes are defined explicitly: geometry only and geometry and topology. The third class, topology only, can be defined implicitly by removing the coordinates from the geometry and topology class of objects. The intended use of these three classes of objects is described in the following clauses.

a) Geometry only - for drawing, display, and geometrically defined operations on raster and vector data structures.

b) Geometry and topology - for vector data structures that use geometric drawing and topological operations.

c) Topology only - for certain analytical operations.

The relationship between the explicitly defined classes of objects and their intended use is specified in Table 2.

Table 2 - Intended use of defined spatial objects

Number of geospatial dimensions	Intended use
	Geometry only (G)	Geometry and topology (GT)
zero dimensions	point1	node
one dimension	line segment	link
	string	chain2
	arc
	G-ring	GT-ring
two dimensions	G-polygon	GT-polygon3
	pixel
	grid cell
three dimensions	voxel

1 There are additional special objects defined for the point of entity point, area point, and label point. 2 There are additional special objects defined for the chain of complete chain, area chain and network chain. 3 There are additional special objects defined for the GT-polygon of universe and void polygon.