[postgis-users] Problem in Polygon-to-Polygon Intersection

[Stephen Woodbridge]

Myunghwa Hwang wrote:
> Hi, list!
> 
> I have a problem in the results of intersection function.
> I have two polygon boundary of a same area for 1990 and 2000,
> and what I want to get is not just an common part but 1990 boundary 
> whose polygons are split by 2000 Boundary,
> like the result of ArcToolbox's Union.
> 
> I used the following query:
> Create Table tb_name with oids as
> select intersection(a.the_geom, b.the_geom)
> from bnd_1990 a, bnd_2000 b
> where intersects(a.the_geom, b.the_geom);
> 
> The problem is the results is missing some minor parts, generating holes 
> in the entire boundary.
> Through some tests, I figured out the missing parts are type of 
> geomertycollection.
> I tried to get right intersected polygon by using GeometryN().
> But some geometries in a geomertycollection were line, not polygon.
> 
> Anybody can tell me how I can get the result from ArcToolbox's Union?
> or how I can avoid the geometrycollection problem?

You can't avoid the geometrycollection, but you can work around it like 
this:

update yourtable set the_geom =
  (select geomunion(geom1) from
    (select (dump(the_geom)).geom as geom1) as foo
     where geometrytype(geom1)='POLYGON'
  )
  where geometrytype(the_geom)='GEOMETRYCOLLECTION';

This will create a new polygon the is the union of the polygons in the 
collection and throw out the points and lines.

Give it a try.

-Steve W.

> Thank you in advance.
> 
> from Myunghwa Hwang_______________________________________________
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