As well as having distinct properties of its own, each of the geometry sub-classes inherits properties from the ST_Geometry supertype. A geometry subtype has one of the following dimensional values:
-1 A value of -1 indicates that the geometry is empty (it does not contain any points).
0 A value of 0 indicates the geometry has no length or area. The subtypes ST_Point and ST_MultiPoint have dimensional values of 0. A point represents a geometric feature that can be represented by a single pair of coordinates, and a cluster of unconnected points represents a multipoint feature.
1 A value of 1 indicates the geometry has length but no area. The set of subtypes that have a dimension of 1 are subtypes of ST_Curve (ST_LineString, ST_CircularString, and ST_CompoundCurve), or collection types containing these types, but no surfaces. In GIS data, these geometries of dimension 1 are used to define linear features such as streams, branching river systems, and road segments.
2 A value of 2 indicates the geometry has area. The set of subtypes that have a dimension of 2 are subtypes of ST_Surface (ST_Polygon and ST_CurvePolygon), or collection types containing these types. Polygons and multipolygons represent geometric features with perimeters that enclose a defined area such as lakes or parks.
A single ST_GeomCollection can contain geometries of different dimensions, and the highest dimension geometry is returned
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