Raster tile size impact results

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Raster tile size impact results

Pierre Bails
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     (result: 46 382 873)
Difference: 35027

DSM px size 20cm : 
raster2pgsql -a -t 250x250 -s 4326 -F pathToDSM.tiff dsmTable  (result:  567 505 888)
raster2pgsql -a -t 50x50 -s 4326 -F pathToDSM.tiff dsmTable      (result: 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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