[postgis-users] 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    : 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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