[postgis-users] Raster tile size impact results

[Pierre Racine]

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] On Behalf
> Of Pierre Bails
> Sent: Wednesday, June 15, 2016 11:41 AM
> To: 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