[postgis-users] Raster pixel value

[Tom van Tilburg]

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 <tom.van.tilburg at gmail.com 
> <mailto:tom.van.tilburg at gmail.com>>
>
>     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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