[postgis-users] area() weighted statistics?

[Jeff Hamann]

I'm trying to create a set of 'unioned' polygons where the statistics 
are the area weighted average from the areas of the adjacenct polygons 
using PostGIS (which rocks, btw) using something like the following code:

SELECT
        ( area( GeomUnion( mat2.the_geom ) ) * mat2.initage ) / sum(
area( GeomUnion( mat2.the_geom ) ) ) as wtinitage,
        GeomUnion( the_geom ) as boundary
FROM mat2, (SELECT mat2.the_geom FROM mat2 ) AS s;


ch1-# ERROR:  aggregate function calls cannot be nested


which would suggest that I cannot perform a query like this:

select area(a.the_geom)*a.attr / sum( area( GeomUnion( a.the_geom ) )

where the denominator is a temp table that contains the unioned 
geometries. I'm thinking I might have to perform this process in a few 
steps. Is that correct, or is there a 'one-liner' method to union a set 
of polygons that touch, such that a set of values can be weighted by 
area of the polygons, before the union? I'm having to to this many steps 
to make sure, I'm getting correct results, but have lose data when I 
geomunion to get a set of touching polygons together.


Thanks
Jeff.


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