[postgis-users] Raster pixel value

[Andreas Forø Tollefsen]

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>

>  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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