[postgis-users] Select polygons within polygon with PostGIS 2.0

[Michal Zimmermann]

Sandro I am definitely going to read up something on PostGIS Topology,
as it sounds really interesting. Thanks for your advice!

On 10/10/12, Sandro Santilli <strk at keybit.net> wrote:
> On Wed, Oct 10, 2012 at 11:44:54AM +0200, Michal Zimmermann wrote:
>> Thanks for the answers guys! I guess the data is ok, I was given them
>> at the university which had bought it from the GIS company, that's why
>> I trust this data source. Also when I try selecting the disctricts
>> with ArcMap, it works (not with QGIS, though!).
>> Sandro, would you be so kind and share some thoughts on PostGIS
>> Topology? As I understood, it is a completely new feature of v2,
>> right? What are the benefits?
>
> One benefit is making the spatial relationships explicit, so that
> you could encode your counties as being composed by the districts,
> and the districts would be composed by faces, which would be defined
> by (shared) edges.
> At that point querying for "all districts in a county" would just be
> a matter of listing the components of a "county" TopoGeometry.
>
> But I guess you're still in the data preparation phase as you still
> don't know which district record belong to which county, right ?
> In that case you'll need ST_Covers (assuming the input data is
> _really_ correct and doesn't contain cases in which the shape
> of a district is slightly outside the shape of its county.
>
> Note that storing the data in a topology would be an excellent way to
> have better control on those kind of data problems (often not easily
> detectable as invisible to the eye).
>
> --strk;
>
>   ,------o-.
>   |   __/  |    Delivering high quality PostGIS 2.0 !
>   |  / 2.0 |    http://strk.keybit.net
>   `-o------'
>
>
>> On 10/10/12, Sandro Santilli <strk at keybit.net> wrote:
>> > On Wed, Oct 10, 2012 at 10:27:42AM +0200, Michal Zimmermann wrote:
>> >> Hi to all,
>> >> I have a relation representing all the counties in my country and the
>> >> other one containing all the districts - e. g. each and every county
>> >> is composed of several districts. How do I select them with spatial
>> >> query? I tried with
>> >> select nazorp from kraje as k, orp_wgs as o WHERE
>> >> ST_Within(o.geom,k.geom)
>> >> AND
>> >> k.nazev = 'Liberecký' // name of the county
>> >>
>> >> but had wrong results returned (well, they are probably not wrong,
>> >> they are just not what I expected them to be). ST_Within only returns
>> >> districts that don't share a boundary with the county, but I need to
>> >> get all the districts within the county. Is that possible? I haven't
>> >> found any built-in function suitable for my needs yet.
>> >
>> > Try ST_Covers, which includes the boundaries.
>> >
>> > PS: yours is the perfect use case for PostGIS Topology !
>> >
>> > --strk;
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-- 
Michal Zimmermann (zimmi)
WWW: http://www.zimmi.cz