[postgis-users] Raster tile size impact results

[Pierre Racine]

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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