Regina,
I'm not convinced ST_Union is the way to go (using ST_Overlaps OR
ST_Intersects as a condition). Basically I want to iterate over the collection
(recursively?) clipping one polygon to another until I'm left with no
overlapping (or intersecting) polygons (planar topology). This is sightly more
complicated than the way I originally posed the problem (I wanted to created
slivers from the overlapping areas to get planar topology).
A little background:
My polygons represent a classification of vegetation of a large area of interest.
In theory any point in the area of interest must fall in one and only one polygon. Due
to an artifact of my image segmentation process, my polygons currently have slight
overlap which I need to "dissolve" (and I don't care which polygon the overlap
sliver gets dissolved into).
Thanks!
Brent.
Paragon Corporation wrote:
One more question - you sure you want ST_Overlaps and not ST_Intersects.
If one geometry sits completely inside another, it is not considered to
overlap, but they do intersect.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Paragon
Corporation
Sent: Wednesday, July 16, 2008 12:41 PM
To: 'PostGIS Users Discussion'
Subject: RE: [postgis-users] Transform overlapping polygons to
non-overlapping?
Brent,
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;
Can be done more efficiently using ST_Dump
INSERT INTO temp3_lines (the_geom)
SELECT ST_ExteriorRing((ST_Dump(the_geom)).geom) AS the_geom FROM
temp2_polys;
Regarding the unioning I mentioned - I thought about more why what I
proposed had still overlapping polygons and I realized its because the
grouping I proposed needs to be A recursive query (which means you'd need to
wrap it in an sql or plpgsql
function) since as it stands it would only find the first root overlaps and
not the A overlap B overlap C (e.g. c would not be in the same grouping and
A,B if it doesn't also overlap with A) .
So two ways
1) Write recursive query (using a plpgsql or sql helper function) - which I
haven't given much thought to the most efficient way of doing that
Or
2) Repeat the union thing I mentioned over and over again until you have a
set that has no more overlapping polygons.
Then you do a ST_Dump to get back individual polygons.
Hope that helps,
Regina
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Brent
Fraser
Sent: Wednesday, July 16, 2008 12:13 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Transform overlapping polygons to
non-overlapping?
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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