I read the original request as asking how to create a non-overlapping
coverage from a set of overlapping polygons, and attribute each face of
the coverage with the sum of its parent polygons' scores. I think this
is different to your solution.
AFAIK the best way to create a non-overlapping coverage from a set of
overlapping polygons is to extract the linework (boundaries) from the
polygons, node it together (by unioning all the lines) and then
re-polygonize. A point-in-polygon for each new face against the
original set can be used to determine the parents, and then the summing
is easy.
Obe, Regina wrote:
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: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] 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: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] 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.
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
-----------------------------------------
The substance of this message, including any attachments, may be
confidential, legally privileged and/or exempt from disclosure
pursuant to Massachusetts law. It is intended
solely for the addressee. If you received this in error, please
contact the sender and delete the material from any computer.
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
