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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