[postgis-users] summarizing a polygon values in a raster

[Peter Tittmann]

Thanks for your suggestion Pierre. I am working with v2.0 and it seems that
ST_PixelsAsPolygons() is not available. The singular Pixel version works
though. Any idea why that would be?

Peter

On Wed, Apr 4, 2012 at 7:31 AM, Pierre Racine
<Pierre.Racine at sbf.ulaval.ca>wrote:

> > I would like to create a raster of an area-weighted average value
> derived from a
> > polygon layer. The raster template is a 2km grid and the polygon layer
> is also a
> > grid but at 10km. If some one could recommend a (high level) strategy
> for this if
> > be grateful.
>
> 1) Create a vector grid having a x and a y like this:
>
> CREATE TABLE yourgrid AS
> SELECT (gvxy).geom geom, (gvxy).x x,  (gvxy).y y
> FROM (SELECT ST_PixelsAsPolygons(rast) gvxy
>             FROM (SELECT ST_MakeEmptyRaster(your own parameters here
> covering your polygon extent)) foo1) foo2
>


>
> 2) Make sure your polygon layer is indexed
>
> CREATE INDEX yourpolylayer_geom_idx  ON yourpolylayer USING gist  (geom );
>
> 3) Intersect mygrid with your polygon layer and generate summary stats at
> the same time:
>
> CREATE TABLE weightedvectorgrid AS
> SELECT g.x, g.y, g.geom, (aws).weightedmean mean
> FROM (SELECT ST_AreaWeightedSummaryStats(intgeom, p.value) aws, g.x, g.y,
> g.geom
>      FROM (SELECT ST_Intersection(g.geom, p.geom) intgeom, p.value, g.x,
> g.y, g.geom
>            FROM yourgrid g, yourpolylayer p
>            WHERE ST_Intersects(g.geom, p.geom)
>           ) foo
>      GROUP BY g.x, g.y, g.geom
>     ) foo2
>
> The ST_AreaWeightedSummaryStats() function is not part of the PostGIS
> release but is available as part of the source tree:
>
>
> http://trac.osgeo.org/postgis/browser/trunk/raster/scripts/plpgsql/st_areaweightedsummarystats.sql
>
> 4) Union all the geometry as a raster (this one might be a bit slow
> depending on the size of your grid)
>
> CREATE TABLE weightedrastergrid AS
> SELECT ST_Union(ST_AsRaster(geom, 'MEAN')) rast
> FROM weightedvectorgrid
>
> I wrote all these without testing so you might have some adjustment to do.
>
> Let me know about your progress...
>
> Pierre
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>
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