[postgis-users] Raster tile size impact results

[Pierre Bails]

Hi,

Thank you for the tips.

Well, I don't know why, but the st_clip function doesn't seem work fine.
After clipping and dumping each pixel as point, some points follow tiles,
others follow the geometry. Any idea why ?
For now, to avoid the problem, I apply a St_ContainsProperly after the dump.

[image: unnamed.png]
Red : geometry
Light blue : tiles
Dark blue : pixels as points

Thank you for your help !

Pierre


Le ven. 15 juil. 2016 à 17:28, Pierre Racine <Pierre.Racine at sbf.ulaval.ca>
a écrit :

> Sorry for the late answer.
>
> You have to vectorize them on the fly from within OpenJump:
>
> SELECT (ST_DumpAsPolygons(rast).* FROM yourrastertable;
>
> If your raster is very big and tiled, just vectorize the tiles you need:
>
> SELECT rid, rast::geometry FROM yourrastertable;
>
> Look at the rids and then:
>
> SELECT (ST_DumpAsPolygons(rast).* FROM yourrastertable
> WHERE rid = x or rid = y
>
> Pierre
>
> > -----Original Message-----
> > From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On
> Behalf
> > Of Pierre Bails
> > Sent: Friday, June 24, 2016 12:32 PM
> > To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> > Subject: Re: [postgis-users] Raster tile size impact results
> >
> > Hello,
> >
> > Unfortunatly, I don't find a way to print raster results in OpenJump.
> Can you
> > explain me how I can do it ?
> >
> > Thank you for your help !
> >
> > Pierre
> >
> > Le lun. 20 juin 2016 à 16:16, Pierre Racine <Pierre.Racine at sbf.ulaval.ca
> > <mailto:Pierre.Racine at sbf.ulaval.ca> > a écrit :
> >
> >
> >       I would try to understand, visually using OpenJump, how your tiles
> are
> > clipped in the two cases... I guess sometimes 1 pixel wide tiles get
> clipped in a
> > strange way.
> >
> >       Pierre
> >
> >       > -----Original Message-----
> >       > From: postgis-users [mailto:
> postgis-users-bounces at lists.osgeo.org
> > <mailto:postgis-users-bounces at lists.osgeo.org> ] On Behalf
> >       > Of Pierre Bails
> >       > Sent: Wednesday, June 15, 2016 11:41 AM
> >       > To: postgis-users at lists.osgeo.org <mailto:postgis-
> > users at lists.osgeo.org>
> >       > Subject: [postgis-users] Raster tile size impact results
> >       >
> >       > Hi all,
> >       >
> >       > I'm looking for get all pixel's value in a raster insert thanks
> to
> > raster2pgsql
> >       > intersect by a polygon.
> >       > Nevertheless, there are some things that I don't understand:
> depends
> > on tile
> >       > size, I don't have the same result... The error between 2 raster
> is also
> > correlate
> >       > to the pixel size (I assume it's due to the pixel number).
> >       >
> >       > Can you explain me how the tile size can influence the final
> result ?
> >       >
> >       > Here is the query :
> >       > SELECT DISTINCT SUM((px).val)
> >       > FROM(
> >       >   SELECT *
> >       >   FROM  mns INNER JOIN
> >       >
> >
> st_setsrid(st_geomfromgeojson('{"type":"Polygon","coordinates":[[[x,y],[x,y],[x
> >       > ,y] ]]}'), 4326) as geom on ST_Intersects(mns.rast, geom)
> >       > ) AS clip, ST_PixelAsCentroids (ST_Clip(clip.rast, clip.geom),1)
> AS px;
> >       >
> >       >
> >       > DSM px size 70cm:
> >       > raster2pgsql -a -t 250x250 -s 4326 -F pathToDSM.tiff dsmTable
> (result:
> > 46 347
> >       > 846)
> >       >
> >       > raster2pgsql -a -t 50x50 -s 4326 -F pathToDSM.tiff dsmTable    :
> 46 382
> > 873
> >       > Difference: 35027
> >       >
> >       >
> >       > DSM px size 20cm :
> >       > raster2pgsql -a -t 250x250 -s 4326 -F pathToDSM.tiff dsmTable :
> 567
> > 505 888
> >       >
> >       > raster2pgsql -a -t 50x50 -s 4326 -F pathToDSM.tiff dsmTable    :
> 596
> > 897 610
> >       > Difference: 29 391 722
> >       >
> >       >
> >       > If we considere 250x250's result as reference, there is a
> difference of
> > 0.07% in
> >       > the first case, and 5% in the second. If the polygon area
> increases, the
> > error rate
> >       > increases.
> >       >
> >       > Thank you,
> >       >
> >       > Pierre
> >       _______________________________________________
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> postgis-users at lists.osgeo.org>
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>
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