[postgis-users] Difference between Polygon and Multipolygon

[Brent Wood]

--- Kai Behncke <kbehncke at igf.uni-osnabrueck.de> wrote:

> Dear list,
> 
> could anybody please give me a practical example where it`s useful to use
> MULTIPOLYGON instead of POLYGON?

3 that I've come across.

A random stratified fisheries survey. The survey area is divided into strata.
In some cases a single strata can comprise disjoint polygons, thus a
multipolygon. Each may have internal regions of foul ground which cannot be
trawled, so each polygon may have its own "holes".

A road dataset, a single road feature may be interrupted by a geographical
feature. The road may be reperesented by a multipolygon, or it's centreline by
a multiline feature.

A simplified lake dataset, for quick retrival & plotting by a web map server,
it can be convenient to have a multipolygon comprising all lakes > 10 sq km in
an optimised data warehouse.  


> I wonder where the difference is and I don`t know where it`s useful to use
> MULTIPOLYGON.

Anywhere you need to address multiple polygons as a single record or single
feature.


No doubt others will have different examples :-)

HTH,

  Brent Wood