[postgis-users] Question about st_difference

[Chris Hermansen]

In addition to Nicolas' suggestion, one thing that can work, for example
when the goal is to "make holes" with one theme in another, is to st_union
the two themes, then re-attribute, then delete the unwanted polygons.  This
has worked for me in relatively small datasets but I've never truly stress
tested it.

2011/9/7 Nicolas Ribot <nicolas.ribot at gmail.com>

> > Hello Group,
> >
> > About st_difference I've seen several topics but unfortunately with none
> of them I could solve my problem.
> >
> > What I want to do is subtract one layer from the other. I could simplify
> this down to the following;
> >
> > Tbl_a contains 1 record
> > "POLYGON(((168119.443682473 451093.811197312,197555.469699649
> 451093.811197312,197555.469699649 437539.54805452,168119.443682473
> 437539.54805452,168119.443682473 437539.54805452,168119.443682473
> 451093.811197312)))"
> >
> > Tbl_b contains 2 records of which both intersects with the object in
> tbl_a
> >
> > "POLYGON(((171424.484910418 453680.34544058,171424.484910418
> 435697.56759188,174568.140889894 435697.56759188,174568.140889894
> 453680.34544058,174568.140889894 453680.34544058,171424.484910418
> 453680.34544058)))"
> >
> > "POLYGON(((184904.90800885 453627.063135843,184904.90800885
> 435644.285287143,188048.563988327 435644.285287143,188048.563988327
> 453627.063135843,188048.563988327 453627.063135843,184904.90800885
> 453627.063135843)))"
> >
> > The result I'm looking for is the object of tbl_a abstracted with both
> objects from tbl_b, since they are both intersecting. In this example the
> result should be 3 squares since it is cut twice by tbl_b.
> >
> >
> > Does anyone know a solution for this?
> >
> > Many thanks in advance,
> >
> > Ge
>
> Hi Ge,
>
> You may want to union objects in table B to create a single polygon,
> then perfom the difference:
>
>
> select st_difference(a.geom, b.geom)
> from
> (select 'POLYGON((168119.443682473 451093.811197312,197555.469699649
> 451093.811197312,197555.469699649 437539.54805452,168119.443682473
> 437539.54805452,168119.443682473 437539.54805452,168119.443682473
> 451093.811197312))'::geometry as geom) as a,
> (
> select st_union(geom) as geom
> from (
>        select 'POLYGON((171424.484910418 453680.34544058,171424.484910418
> 435697.56759188,174568.140889894 435697.56759188,174568.140889894
> 453680.34544058,174568.140889894 453680.34544058,171424.484910418
> 453680.34544058))'::geometry as geom
>        UNION
>        select 'POLYGON((184904.90800885 453627.063135843,184904.90800885
> 435644.285287143,188048.563988327 435644.285287143,188048.563988327
> 453627.063135843,188048.563988327 453627.063135843,184904.90800885
> 453627.063135843))'::geometry as geom
> ) as foo
> ) as b;
>
> (see attached pictures)
>
> Nicolas
>
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>
>


-- 
Chris Hermansen
*Vice President*

TECO Natural Resource Group Limited
301 · 958 West 8th Avenue
Vancouver BC CANADA · V5Z 1E5
Tel +1.604.714.2878 · Cel +1.778.840.4625
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