[postgis-users] Raster pixel value

[Andreas Forø Tollefsen]

A small note regarding this issue.

My problem is that I never get a mean value of 1 even if all pixels inside
the geometry is one.

Could this be because: 6x6 pixels goes into one polygon when visually
controlling. If each pixel has the value 1, then this will be calculated as
36 / 36 = 1. However, if it calculates the sum to be 36 and divide by a
number higher than 36 pixels, then the result will always be below 1.
What i am thinking is that while it sums up the pixel values correctly, it
does not count only the 36 pixels, but also neighboring pixels. Therefore:
1+1+1+1...n36 / Number of pixels higher than 36 will always lead to a
number lower than 1.

Anyone who knows the functions well could probably answer this.

Best regards,
Andreas


2011/11/25 Andreas Forø Tollefsen <andreasft at gmail.com>

> Could this have to do with the tiling of the raster?
> I will try to run the same query with a untiled mountain raster to see if
> that changes anything.
>
> Btw. When loading a tiled postgis raster into qgis it shows up with many
> artifacts and no data areas. The same raster untiled does not show up the
> same way.
> Qgis bug?
>
> Andreas
>
> 2011/11/25 Andreas Forø Tollefsen <andreasft at gmail.com>
>
>> Hi,
>>
>> Thanks for all of the suggestions. I will do some more testing. However,
>> as for suggestion 1 i think the pixel size should be the same as the
>> original raster or am I wrong?
>>
>> Both the mean_mnt_bin raster and the priogrid_land shapefile can be
>> downloaded as zip (2 mb) here:
>> http://gisintersect.com/mean_mnt_bin.zip
>> http://gisintersect.com/priogrid_land.zip
>>
>> Any help on getting the correct values would be very much appreciated.
>>
>> My query:
>> DROP TABLE IF EXISTS mountain_cell;
>>
>> SELECT
>> a.gid As id,
>> (ST_SummaryStats((ST_Union(ST_MapAlgebraExpr(ST_AsRaster(a.cell, b.rast,
>> '32BF'), b.rast, 'rast2', '32BF','INTERSECTION','0','0',0))).rast,
>> false)).mean As avgmnt
>> INTO mountain_cell
>> FROM
>> priogrid_land a LEFT JOIN
>> mountain b
>>     ON ST_Intersects(a.cell, b.rast)
>> GROUP BY a.gid
>> ORDER BY a.gid;
>>
>
>
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