Andreas,
Would it be possible to share code and/or sample data?
With me St_SummaryStats seems to work correct, even when I set all my
raster values to 1.
Chrs,
Tom
On 24-11-2011 13:42, Andreas Forø Tollefsen wrote:
I double checked the values that I get from this query and they do not
seem to be correct.
For polygon cells that have only pixels with the value 1 inside only
have a mean of 0.85 (see image).
The correct value should be a mean of 1. After some visual controls I
am not sure if these numbers are correct.
There are many polygon cells with only pixel value 1, but no polygon
cell with a higher mean than .87.
Any idea why this could be? Both the raster and polygons are in srid 4326.
Andreas
2011/11/24 Andreas Forø Tollefsen <[email protected]
<mailto:[email protected]>>
I know. RTFM before asking dumb questions. Set the
ST_Summary(rast, false) so it does not exclude_nodata_value? Right?
Andreas
2011/11/24 Andreas Forø Tollefsen <[email protected]
<mailto:[email protected]>>
Great. I updated to latest rev and added the st_union.sql to
my function list. Now it works.
However, I need to ask one more question.
When running the script, it calculates the average pixel value
inside the polygon. It does not take into account the NULL
DATA values.
Is there any way i can make the script calculate the average
pixel value but count NULL DATA values as 0?
For instance, one pixel in a cell belonging to the Comoros
have a value (0.95). The rest is NULL. Hence, it seems a bit
wrong that the whole polygon get 0.95 when almost none of its
area have any elevation?
Any ideas?
Thank you so much for this info Tom!
Andreas
2011/11/24 Tom van Tilburg <[email protected]
<mailto:[email protected]>>
Yes, I think so.
Current windows build is *r8221
*
Also, I think ST_Union(raster) (which I used in the
example) is not included in this version yet.
You would have to download a prototype from.
http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql/st_union.sql
Chrs,
Tom
On 24-11-2011 12:07, Andreas Forø Tollefsen wrote:
Great, Thank you so much for this.
However, I do not seem to have the
ST_MapAlgebraExpr(rast, rast.....) function. Was this
implemented in rev 8001?
Andreas
2011/11/24 Tom van Tilburg <[email protected]
<mailto:[email protected]>>
Andreas,
We did approx. the same thing for non-quadrate
polygons. Perhaps it might be useful:
Step 1: Make a raster from every polygon, based on
the grid specifications of the elevation raster. Here
is also the solution: the raster cells will only be
created for cells that have their midpoint *inside*
your geometry.
ST_AsRaster(a.geom, b.rast, '<pixtype>')
Step 2: Overlay the elevation raster with the raster
you just created and keep only the values of the
elevation raster
ST_MapAlgebraExpr(
<raster from geometry>
,b.rast
,'rast2' -- <-- keep only raster 2 value
, '<pixtype>','INTERSECTION','0','0',0
)
Step 3: get the mean from the statistics on the
resulting raster
(ST_SummaryStats(
(ST_Union( -- < --- we did a UNION because we
occasionaly had vectors crossing tiled rasters
<overlay raster>
)).rast
)).mean As avg_height
That did the trick. Complete script is below.
I suspect your method of doing a ST_Intersection for
every pix. makes it slower because it creates a
geometry first that you do not really need.
Cheers,
Tom
----------
FULL SCRIPT
SELECT
a.gid As id,
(ST_SummaryStats(
(ST_Union(
ST_MapAlgebraExpr(
ST_AsRaster(a.geom, b.rast, '32BF')
,b.rast
,'rast2', '32BF','INTERSECTION','0','0',0
)
)).rast
)).mean As avg_height
FROM
polygons.grid a LEFT JOIN
rasters.elev b
ON ST_Intersects(a.geom, b.rast)
GROUP BY a.gid
On 24-11-2011 11:14, Andreas Forø Tollefsen wrote:
Hi,
I am trying to calculate the average pixel value in
a elevation raster inside quadrate polygons.
However, I am not getting the correct values from my
query:
SELECT gid, AVG(((foo.geomval).val)) as avgmnt
FROM (SELECT p.gid, ST_Intersection(p.cell, r.rast)
AS geomval FROM mountain r, priogrid_land p WHERE
ST_Intersects(p.cell, r.rast, ) AND p.gid =186124)
AS foo
GROUP BY gid ORDER BY gid;
The problem here is that the ST_Intersects(geom,
rast) takes into consideration the pixels that is
outside, but touches the border of the quadrate
polygons. Then, the average values for each quadrate
polygon is affected by pixels inside other polygons.
This will potentially lead to a flawed result.
So what I want is to be able to calculate the
average value for the pixels INSIDE the polygon
excluding those outside.
How can i restrict the AVG pixel value to be
calculated only for pixels that is inside the
polygon, and not the pixels that touch the outside
of the border?
Thanks!
Best,
Andreas
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