[postgis-users] Raster pixel count too high

[Andreas Forø Tollefsen]

Hi Kim,

Thanks for your answer. However, we want raster as an output, since we want
to be able to use the summarystats function.
Please elaborate how you think ST_PixelAsPolygons should solve out issue?
Thanks.

Andreas

2013/6/18 Kim Bisgaard <kib at dmi.dk>

>  Hi,
>
> Try to use 'ST_PixelAsPolygons(ST_Clip(n.rast, p.cell))'
> instead of 'ST_Intersects(n.rast, p.cell)'
>
> Regards,
> Kim
>
>
> On 2013-06-18 11:03, Andreas Forø Tollefsen wrote:
>
> Hi,
>
>  We are working on a raster summarystats script to calculate various
> statistics for the pixels within fishnet polygons.
>
>  Our raster cell size is 0.0083333333333... x 0.0083333333333... degrees
> while our quadrat polygons are 0.5 x 0.5 decimal degrees.
> This should give us 60x60 raster pixels within each of our polygons.
> ArcGIS zonal statistics returns a pixel count of 3600 in addition to other
> statistics.
> However, PostGIS returns 3721 pixel count.
>
>  We do not really understand why, but it seems that our query includes
> some pixels that are outside of the polygon, but still touches the vertices
> of the polygon and are therefore included in the calculation.
> Are there any way of modifying our script to return the same result as
> ArcGIS?
> Thanks!
>
>  Andreas
>
>  script:
>
>  /* This query makes one raster for each PRIO-GRID cell. Clip and union
> is the procedure. */
> INSERT INTO nightlightsprio (gid, "year", rast)
> (SELECT gid, "year", ST_Union(raster) as rast
> FROM
> (SELECT p.gid, n."year", ST_Clip(n.rast, p.cell) as raster
> FROM nightlights n, priogridyear p
> WHERE ST_Intersects(n.rast, p.cell)
> AND n."year" = p."year"
> )
> as priorast
> GROUP BY gid, "year");
>
>
>  /* Default BandNoDataValue is 0. Raster value 0 means no light, not no
> data. Setting to NULL. This produces correct results. */
> UPDATE nightlightsprio2 SET rast = ST_SetBandNoDataValue(rast, 1, NULL);
>
>
>  ALTER TABLE nightlightsprio2 ADD COLUMN nightlights_sum double precision,
> ADD COLUMN nightlights_mean double precision,
> ADD COLUMN nightlights_sd double precision,
> ADD COLUMN nightlights_min double precision,
> ADD COLUMN nightlights_max double precision,
> ADD COLUMN nightlights_count integer;
>
>  UPDATE nightlightsprio2 SET nightlights_sum =
> (ST_SummaryStats(rast)).sum;
> UPDATE nightlightsprio2 SET nightlights_mean =
> (ST_SummaryStats(rast)).mean;
> UPDATE nightlightsprio2 SET nightlights_sd =
> (ST_SummaryStats(rast)).stddev;
> UPDATE nightlightsprio2 SET nightlights_min = (ST_SummaryStats(rast)).min;
> UPDATE nightlightsprio2 SET nightlights_max = (ST_SummaryStats(rast)).max;
> UPDATE nightlightsprio2 SET nightlights_count =
> (ST_SummaryStats(rast)).count;
>
>
>
> _______________________________________________
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>
>
> --
> Kim Bisgaard
>
> Application Development Division     Phone: +45 3915 7562 (direct)
> Danish Meteorological Institute      Fax: +45 3915 7460 (division)
>
>
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