The ST_Polygon data type is a two-dimensional surface stored
as a sequence of points defining its exterior bounding ring and 0
or more interior rings. Most often, ST_Polygon defines parcels of
land, water bodies, and other features having spatial extent.
Properties
The ST_Polygon is always simple.
The exterior and any interior rings define the boundary of an ST_Polygon,
and the space enclosed between the rings defines the interior of ST_Polygon.
The rings of an ST_Polygon can intersect at a tangent point, but never
cross. In addition to the other properties inherited from the superclass,
ST_Geometry, ST_Polygon has area.
The following figure shows
examples of ST_Polygon objects: (1) is an ST_Polygon whose boundary
is defined by an exterior ring; (2) is an ST_Polygon whose boundary
is defined by an exterior ring and two interior rings. The area inside
the interior rings is part of the ST_Polygon's exterior; (3)
is a legal ST_Polygon because the rings intersect at a single tangent
point.Figure 1: Examples of
ST_Polygon objects
Functions
The following functions operate
on ST_Polygon:
ST_Area()
The function returns the polygon's area as a double-precision
number.
ST_ExteriorRing()
The function returns the polygon's exterior ring as an ST_LineString.
ST_NumInteriorRing()
The function returns the number of interior rings that the polygon
contains.
ST_InteriorRingN()
The function takes an ST_Polygon and an index and returns the nth
interior ring as an ST_LineString.
ST_Centroid()
The function returns an ST_Point that is the center of the ST_Polygon's
envelope.
ST_PointOnSurface()
The function returns an ST_Point that is guaranteed to be on the
surface of the ST_Polygon.
ST_Perimeter()
The function returns the perimeter of an ST_Polygon or ST_MultiPolygon.
