[postgis-users] Raster tile size impact results

[Pierre Racine]

I will need to see your last query to try to find the problem.

Pierre

From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of Pierre Bails
Sent: Wednesday, July 20, 2016 5:36 AM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: Re: [postgis-users] Raster tile size impact results

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.

[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<mailto: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<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<mailto: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>
> <mailto: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>
> <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> <mailto:postgis-<mailto:postgis->
> users at lists.osgeo.org<mailto: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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