[postgis-users] "summing" (polygon, float) pairs

[Obe, Regina]

If the below conditions are true, then I would say you can do this in
PostGIS.

A solution that comes to mind is this

SELECT Sum(score) as totscore, ST_Union(the_geom) as newgeom
FROM poly
GROUP BY CAST(random()*10000 As int)
HAVING All_Disjoint(the_geom)


Set the 10000 so some function of the size of your set.  You always want
it to be lower than the number of polygons you have. The larger the
number the fewer polygons you would expect per match.

All_Disjoint is a yet to be defined aggregate function you will need to
create yourself that  checks each
incoming geometry against the previous set and returns true if it is
disjoint (not intersecting) any of the previous.

Unfortunately the above will not use indexes.

Aggregates are pretty easy to create in PostgreSQL so I'll leave that as
an exercise for you to figure out.  The answers to the above will
probably give different answers each time since it does a random walk.

Hope that helps,
Regina





-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Obe,
Regina
Sent: Thursday, October 02, 2008 10:28 AM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] "summing" (polygon, float) pairs

Reid,

I assume
1) You don't care if polygons are reused to form your new sets 
2) You don't care how many polygons are in each set.  
3) You also are not looking for all possible permutations - just a
random set of answers that solve your equation?



-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Reid
Priedhorsky
Sent: Wednesday, October 01, 2008 6:35 PM
To: PostGIS Users Discussion
Subject: [postgis-users] "summing" (polygon, float) pairs

Dear all,

I have a set of polygons. Associated with each polygon is a number; call

it score. Each polygon can intersect with zero or more other polygons.

What I would like to compute is a set of non-intersecting polygons with 
summed scores -- each of these polygons would be a set of points where 
the sum of all the original polygons' scores covered by the result 
polygon are equal.

Here's an example in one dimension:

Input:

----3------
      ----1----
         ----4----
                     ----2----

Output:

--3--
      -4-
         -8-
            -5-
               -4-
                     ----2-----

It is OK if two adjacent result polygons have equal sums.

Can this be done in PostGIS?

Reid

p.s. thanks for the help on my question a few days ago! I felt a tad 
foolish after learning it was just a missing isvalid() constraint. I 
thought I had one but didn't look carefully enough.
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