Regina,
The "SELECT MAX..." query didn't work on my sub-set of 12800 polygons. It
created 12643 polygons some of which overlap (I expected more, not less, than the
original).
I may try converting to linestrings, creating one "minimum bounding rectangle"
for the entire dataset, then doing an intersect of the lines with the MBR. In my case
this would be ok as there are not attributes on the polygons yet.
Thanks!
Brent
Obe, Regina wrote:
Brent,
I guess it really depends on how exactly you want to achieve
non-overlapping.
If for example you are basing it on some sort of attribute and all your
overlapping polygons are valid
Then a simple
SELECT ST_Union(the_geom) As newgeom, field1
FROM sometable
GROUP BY field1
I think will guarantee non-overlapping polygons because as part of the
process of ST_Union - it would irradicate the overlapping regions to
just create one. That is part of the reason why its so much slower than
ST_Collect for example.
For your exact case below - you would union all the overlapping polygons
together which could be really slow depending on how many overlap. The
query I would write to achieve that would be something like this
SELECT MAX(a.gid) As newgid, ST_Union(a.the_geom) As the_geom
FROM poly a
GROUP BY (SELECT MAX(r.gid) FROM poly r
WHERE (a.gid = r.gid OR ST_Overlaps(r.the_geom, a.the_geom)));
Hope that helps,
Regina
------------------------------------------------------------------------
*From:* [EMAIL PROTECTED] on behalf of Brent
Fraser
*Sent:* Fri 7/11/2008 12:14 PM
*To:* PostGIS Users Discussion
*Subject:* Re: [postgis-users] Transform overlapping polygons to
non-overlapping?
To All,
There doesn't seem to be an obvious answer to the problem given below
(aka cleaning polygons, creating planar polygons, etc). I did see a
note on the PostGIS wiki wishlist to "Add a geometry cleaner". There is
also a suggestion to convert to linestrings, node, then polygonize
(while that may work for a small set of polygons, I've got 1.1 million
to clean). JTS, Geos, etc will likely fail due to the large number of
polygons so I'll need a different approach.
I'm considering writing some code to iterate through my table of
polygons, cleaning a small subset at a time. I think using PostGIS for
the geometry storage and spatial query/selection makes sense. Any
suggestions on which API to use?
GDAL's OGR
PostgreSQL's libpq
other?
Thanks!
Brent Fraser
Brent Fraser wrote:
PostGIS'ers,
I've got a table of overlapping polygons. How can I make it a table of
non-overlapping polygons?
For example, if table "polys2" contains two polygons A1 and B1 which
overlap. I'd like to create table "polys3" with polygons A2, B2, C2,
where C2 is the overlap region of A1 and B1, and A2 = A1 - C2, and B2 =
B1 - C2.
Looking at the overlay operations in the JTS doc it looks like doing an
Intersection (to get only the overlapping area) then adding the
Symmetric Difference (to get the non-overlapping areas) might work.
Am I on the right track or is there an easier way (since all the
polygons are in one table)?
Thanks!
Brent Fraser
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