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List:       postgis-users
Subject:    Re: [postgis-users] Raster cell size and wrong mean value calculation problem
From:       Simon Appelt <flansch () gmx ! de>
Date:       2013-01-23 20:08:43
Message-ID: 5100434B.1060601 () gmx ! de
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Hi Bborie,

thanks for your fast answer! Wow, i really missed to read that :-(
But my second query ,which i posted below, provides the same Mean-Value 
result as the first. If i understand you correct, the st_clip command 
should end up with a different result .

Well i'll do some further testing tomorrow and keep the list up to date.

Simon


Am 23.01.2013 17:05, schrieb Bborie Park:
> By the looks of the docs for ST_Intersection, what you're getting is
> correct.  The second paragraph indicates that the raster gets
> vectorized using ST_DumpAsPolygon() which does merge neighboring cells
> with the same value in one larger geometry.
> 
> http://postgis.net/docs/manual-2.0/RT_ST_Intersection.html
> 
> The query using ST_Clip is more appropriate.
> 
> -bborie
> 
> On Wed, Jan 23, 2013 at 6:33 AM,  <fLaNsch@gmx.de> wrote:
> > Hi Postgis User-list,
> > 
> > i've downloaded and migrated some srtm3 rasterdata into my PostgreSQL/Postgis 2.0 \
> > Database successfully. Now i would like to calculate the mean elevation value of \
> > a given polygon area by intersecting my polygon layer with the raster grid. 
> > Using a SQL-Statement, which was shown by Pierre Racine at the FOSS4G 2011:
> > 
> > SELECT polyID,
> > (ST_Intersection(geom, rast)).geom poly,
> > (ST_Intersection(geom, rast)).val elevation
> > FROM polygon, srtm WHERE ST_Intersects(geom, rast);
> > 
> > 
> > What i expected the polygon-result geometry would look like:
> > _ _ _ _
> > > _|_|_|_|
> > > _|_|_|_|
> > > _|_|_|_|
> > > _|_|_|_|
> > 
> > What the result actually looks like:
> > _ _ _ _
> > > ___|_  |
> > > _  |_| |
> > > _|___|_|
> > > _|_|_|_|
> > 
> > 
> > The raster cells with the same elevation value got merged, why?
> > The mean value which based on the actual result cannot be correct. Same result \
> > different SQL-Statement without selecting the geometry: 
> > Select polyid,(stats).mean from (Select  \
> > a.polyid,st_summarystats(st_clip(b.rast,1,a.geom))as stats from polygon a, srtm b \
> > where ST_intersects(a.geom,b.rast ) 
> > 
> > I'm very grateful for any help!
> > 
> > Best regards,
> > Simon
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users@lists.osgeo.org
> > http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
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