To all,
My quest for non-overlapping polygons continues:
I started with a table (temp_polys) of 3253 polygons (with some overlap) with a
"class" attribute.
To get rid of overlapping polys with the same class value:
CREATE TABLE temp2_polys as SELECT class, ST_UNION(the_geom) from
temp_polys GROUP BY class;
This created a table of 32 multi-polygons (grouped by class). I still have to
remove the overlap between polygons with different class values, so my plan is
to convert to linestrings, node the linestrings, polygonize, and (re)assign the
class value using StarSpan. So first:
Convert to linestrings:
INSERT INTO temp3_lines (the_geom) SELECT ST_ExteriorRing(
ST_GeometryN(the_geom, generate_series(1, ST_NumGeometries(the_geom)))) AS
the_geom FROM temp2_polys;
This produced 1768 linestring records. Attempting to node the linestrings:
INSERT INTO temp4_lines (the_geom) SELECT St_Union(the_geom) AS
the_geom FROM temp3_lines;
Yikes! This query ran for 4.5 hours and crashed Postgres (1.8 gHz Windows XP,
Postgres 8.3.3, PostGIS 1.3.3).
I dumped the temp3_lines table into a shapefile and asked OpenJump to node AND
polygonize. That took 24 seconds.
Since the above data is a small sub-set of my 1.2 million polygons, OpenJump is
not really a solution for cleaning the data all at once. Looks like some
scripting is in order...
Brent
Brent Fraser wrote:
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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