[gdal-dev] Differences in behavior between pgsql query and use of GDAL API

[Alexandre Gacon]

Hi Luca,

I follow your advice and now I have all my features. I had to do some
union/collect to have the awaited result.

It's just sad that I can't take more advantage of GDAL to do this : do we
know why my first method can't work ? Because of a limitation in
ST_GeomFromText ? Because of a limitation in GDAL ? Should I used WKB
instead of WKT to have the same result ?

Regards,

Alexandre Gacon

2011/6/13 Luca Sigfrido Percich <sigfrido at tiscali.it>

> Hi Alexandre,
>
> if I well understood your problem, you're already doing half of the job
> (the queries)in PostGIS, so why not doing everything using PostGIS
> queries?
> Moreover, I think you don't need union.
>
> Assuming towns is a polygon layer, try:
>
> select distinct t.* from
>  towns as t,
>  (select distinct * from towns where town_in_group_A_condition) as t2
>  where t.fid = t2.fid or touches(t.geom, t2.geom);
>
> Let us know.
>
> All the best
>
> Sig
>
>
> Il giorno lun, 13/06/2011 alle 09.52 +0200, Alexandre Gacon ha scritto:
> > Hi,
> >
> >
> > I need to do the following request to find some data:
> >
> >
> > - I have a bunch of towns (let's call them group A)
> > - I retrieve the towns touching the group A (let's call them group B)
> > - I make the union of group A and group B
> > - I ask the towns covered by the resulting union
> >
> >
> > For the moment, I use GDAL to do the processing like that:
> >
> >
> > - I use OGRGeometry::Union to make the union of group A towns
> > - I send a request to a postgis database with a ST_Touches where
> > clause and the group A geometry as WKT
> > - I make the union of the resulting geometries with OGRGeometry::Union
> > - I send a request to a postgis database with a ST_Covers where clause
> > and the union of group A and group B as WKT
> >
> >
> > My problem is that I don't retrieve all the towns I initially have to
> > build my geometry.
> >
> >
> > If I try to do the same request totally in pgsql (union, touches and
> > covers), I have the awaited result. If I compare the area of the
> > geometry of the ST_Covers where clause (between generation with GDAL
> > and computation by postgis), I ha ve the same area.
> >
> >
> > What could explain the difference in the result ? Precision loss in
> > GDAL ? Precision loss because of the use of WKT ? Should I use WKB
> > instead of WKT ? Should I do all my computations in postgis ?
> >
> > --
> > Alexandre Gacon
> >
> > _______________________________________________
> > gdal-dev mailing list
> > gdal-dev at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/gdal-dev
>
>
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> PRIVACY
> Le informazioni contenute in questo messaggio sono riservate e
> confidenziali. Il loro utilizzo e' consentito esclusivamente al destinatario
> del messaggio, per le finalità indicate nel messaggio stesso. Qualora Lei
> non fosse la persona a cui il presente messaggio è destinato, La invitiamo
> ad eliminarlo dal Suo Sistema e a distruggere le varie copie o stampe,
> dandone gentilmente comunicazione all’indirizzo mail del mittente. Ogni
> utilizzo improprio e' contrario ai principi del D.lgs 196/03 e alla
> legislazione europea (Direttiva 2002/58/CE).
>



-- 
Alexandre Gacon
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