> 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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