[postgis-users] area() weighted statistics?

[Stephen Woodbridge]

Have you tried doing a sub query that computes the areas and then only 
do the sums in the main query?

-Steve

Jeff Hamann wrote:
> 
> 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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