It sounds like you are after performing an overlay operation on two
polygonal datasets.
I would do something like:
- extract all lines from your polygons
- node the linework
- re-polygonize the noded lines
- transfer attributes from the original dataset to the newly formed
polygons.
http://postgis.refractions.net/support/wiki/index.php?ExamplesOverlayTables
Hope this helps,
Kevin
-------------
Kevin Neufeld
Software Developer
Refractions Research Inc.
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RAVI KUMAR wrote:
Hi,
a and b, are multiple tables derived from -2- shape files saved as
POST-GIS data.
Slide 1:
'a' this data as in the slide show has attributes, This is a square
with polygons.
'b' as in the slide show is made up of 2 polygons with attributes which
overlap 'a'.
Slide 2: a and b are in UNION with the syntax below. But this is not the desired
result. There are polygons overlapping other polygons, one below
the other.
Slide 3:
'abc' is a union of a and b, with desired result
My further analysis with post GIS awaits HELP.
Cheers
Ravi Kumar
"Obe, Regina" <[EMAIL PROTECTED]> wrote:
Ravi,
Is
the graphic one record or multiple records? If multiple records - I'm
afraid you may be asking 2 conflicting questions depending on your
dataset so I'm not sure there is anything that can remedy that aside
from treating them as two separate questions.
I'm also afraid you are getting into
territory I'm very weak. You may want to investigate
use of st_boundary (which will give
you a multilinestring of a polygon/multipolygon boundary and then
apply buildarea to that).
The below should get rid of some redundant
overlapping polygons, but probably won't completely satisfy what you
want to do. I'm also thinking you may want to be using intersection
instead of geomunion for the inner part. But again depends what you
are trying to answer.
INSERT INTO abc(code, info,
the_geom)
SELECT newtb.code, newtb.info, multi(buffer(geomunion(distinct newtb.cgeom), 0.0)) as thenewgeom
FROM
( SELECT a.code, b.info, intersection(a.geometry, b.geometry) as cgeom
FROM a
INNER JOIN b ON a.geometry && b.geometry AND intersects(a.geometry, b.geometry)
UNION ALL
SELECT a.code, null As info, a.geometry as cgeom
FROM a
LEFT JOIN b ON a.geometry && b.geometry AND intersects(a.geometry, b.geometry)
WHERE b.geometry IS NULL
UNION ALL
SELECT null as
code, b.info, b.geometry as cgeom
FROM b LEFT JOIN a ON a.geometry && b.geometry AND intersects(a.geometry, b.geometry)
WHERE a.geometry IS NULL
) AS newtb
Hope that helps,
Regina
Hi Regina,
giving the link to show actual how the UNION of polygons is
not working as per expectation.
Please see the clips in the link.
CREATE TABLE
abc(code smallint, info smallint) with oids;
SELECT
AddGeometryColumn('public', 'abc', 'the_geom', 4326, 'MULTIPOLYGON', 2);
--The insert
INSERT INTO abc(code, info,
the_geom)
SELECT newtb.code, newtb.info, geomunion(newtb.cgeom)
as thenewgeom
FROM
( SELECT a.code, b.info, geomunion(a.geometry, b.geometry) as cgeom
FROM a
INNER JOIN b ON a.geometry && b.geometry AND intersects(a.geometry, b.geometry)
UNION ALL
SELECT a.code, null As info, a.geometry as cgeom
FROM a
LEFT JOIN b ON a.geometry && b.geometry AND intersects(a.geometry, b.geometry)
WHERE b.geometry IS NULL
UNION ALL
SELECT null as code, b.info, b.geometry as cgeom
FROM b LEFT JOIN a ON a.geometry && b.geometry AND intersects(a.geometry, b.geometry)
WHERE a.geometry IS NULL
) AS newtb
May be I should be using different syntax for such a result.
Ravi Kumar
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