[postgis-users] Raster pixel count too high

Hugues François hugues.francois at irstea.fr
Tue Jun 18 06:05:28 PDT 2013


In previous tests we noticed some differences in clipping operations between postgis and arcgis. Are your quadrat polygons *really* aligned with raster's pixels ?


De : postgis-users-bounces at lists.osgeo.org [mailto:postgis-users-bounces at lists.osgeo.org] De la part de Andreas Forø Tollefsen
Envoyé : mardi 18 juin 2013 15:00
À : PostGIS Users Discussion
Objet : Re: [postgis-users] Raster pixel count too high

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?

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

Try to use 'ST_PixelAsPolygons(ST_Clip(n.rast, p.cell))'
instead of 'ST_Intersects(n.rast, p.cell)'


On 2013-06-18 11:03, Andreas Forø Tollefsen wrote:

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?



/* 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
(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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