Andreas,
I didn't have time to reproduce your problem yet. Did you have any
succes by yourself on this issue?
Could it have something to do with counting the non-data values as
value? This is what I experience with a similar function (ST_Clip) that
consequently gave me the value '0' instead of the nodata value. The
result of that is that the mean was often lower than expected.
Perhaps you could rewrite your previous example to something with
auto-generated values in the script. That saves time in reproducing.
Cheers,
Tom
On 25-11-2011 18:11, Andreas Forø Tollefsen wrote:
Update:
I think my suspicion is correct. If I do a ST_Summarystats().sum and
divide this on 36 my MAX value will be 1.
Hence, I think the number of values counted and the number of
observations counted is not equal.
New query:
DROP TABLE IF EXISTS mountain_phil_cell;
SELECT
a.gid As gid,
(ST_SummaryStats((ST_Union(ST_MapAlgebraExpr(ST_AsRaster(a.cell,
b.rast, '32BF'), b.rast, 'rast2',
'32BF','INTERSECTION','0','0',0))).rast, false)).sum / 36 As avgmnt
INTO mountain_phil_cell
FROM
priogrid_land a LEFT JOIN
mountain_phil b
ON ST_Intersects(a.cell, b.rast)
GROUP BY a.gid
ORDER BY a.gid;
2011/11/25 Andreas Forø Tollefsen <[email protected]
<mailto:[email protected]>>
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 <[email protected]
<mailto:[email protected]>>
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 <[email protected]
<mailto:[email protected]>>
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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