User's Guide and Reference

Multipolygons

The boundary of a multipolygon is the cumulative length of its element's exterior and interior rings. The interior of a multipolygon is defined as the cumulative interiors of its element polygons. The boundary of a multipolygon's elements can intersect only at a tangent point. In addition to the other properties inherited from the superclass geometry, multipolygons have area. Multipolygons define features such as a forest stratum or a non-contiguous parcel of land such as an island chain.

Figure 13. Multipolygons.

1. A multipolygon with two polygon elements. The boundary is defined by the two exterior rings and the three interior rings.
2. A multipolygon with two polygon elements. The boundary is defined by the two exterior rings and the two interior rings. The two polygon elements intersect at a tangent point.

Functions that operate on multipolygons:

ST_Area

Takes a multipolygon and returns the cumulative area of its polygon elements as a double precision number. For more information, see ST_Area.

ST_Centroid

Takes a multipolygon and returns a point that is its geometric-weighted center. For more information, see ST_Centroid.

ST_NumGeometries

Takes a homogeneous collection and returns the number of base geometry elements it contains. For more information, see ST_NumGeometries.

ST_GeometryN

Takes a homogeneous collection and an index and returns the nth base geometry. For more information, see ST_GeometryN.