[postgis-users] Raster pixel count too high

[Andreas Forø Tollefsen]

Thanks for your answers.

First to Hugues. I do not think they are perfectly aligned. The raster we
have imported seems to start just west of -180. Hence, it is not within the
limits of SRID 4326. We have to modify it a little.
What I do find strange is that ArcGIS counts 3600 which is the expected
pixel count. So why these two functions count so differently on the exact
same data is weird.
Try yourself with one of the Nightlights data:
http://www.ngdc.noaa.gov/eog/dmsp/downloadV4composites.html and our vector
grid shapefile available here: http://www.prio.no/Data/PRIO-GRID/

Kim: We do not directly use ST_Intersection in our script, but ST_Clip to
clip the raster according to our polygons, and only where they
ST_Intersects(), not ST_Intersection();
I still do not understand your suggestion.

Best,
Andreas

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

>  Hi,
>
> Because ST_Intersection() returns neighbour pixels sharing a same value as
> only one
>  polygon. You thus get a bigger area and thus more points.
>
> Bit me once :-/
>
> Regards,
> Kim
>
>
>
>
>
> On 2013-06-18 14:59, Andreas Forø Tollefsen wrote:
>
> 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;
>>
>>
>>
>>  _______________________________________________
>> postgis-users mailing listpostgis-users at lists.osgeo.orghttp://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>>
>> --
>> Kim Bisgaard
>>
>> Application Development Division     Phone: +45 3915 7562 (direct)
>> Danish Meteorological Institute      Fax: +45 3915 7460 (division)
>>
>>
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>>
>
>
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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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