Very nice Regina,
I think you'll find that if you use "UNION ALL" instead of "UNION" in
your function, you'll get better performance. The latter computes the
set union of the two queries, and in this case, will try to remove
duplicate rows by comparing the bounding boxes of the linear rings. The
former case simply concatenates the results of one query to another.
(Here, the set of exterior rings and interior rings should be distinct
already)
http://www.postgresql.org/docs/8.3/static/sql-select.html#SQL-UNION
Cheers,
Kevin
Obe, Regina wrote:
Take a look at the PostGIS wiki function fnpoly_to_rings and see if it
does something like what you are looking for.
http://postgis.refractions.net/support/wiki/index.php?plpgsqlfunctions
You might need to modify it slightly to create single lines and then the
polygon id of left and right since it maintains the number of records
simply by turning the MULTIPOLYGON into MULTILINESTRINGS which would
require leaving out the ST_Collect step and then doing an SQL update
after the fact based on equality of line geometries.
Only caveate - as some people have mentioned (generate_series degrades
for large numbers - I haven't had big enough geometries to run into the
issue) .
Hope that helps,
Regina
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Lee
Hachadoorian
Sent: Monday, March 10, 2008 1:21 AM
To: [email protected]
Subject: [postgis-users] Turn a Polygon into Lines
I'm looking through the PostGIS reference, and I can't seem to find a
way to take a geometry of polygons and turn it into lines. What I'm
looking for is something like the ArcGIS Feature to Line geoprocessor,
which will create a line shapefile where each feature is an arc
representing the boundary between neighboring polygons with a field
indicating the ids of the polygon on either side.
Functions like ST_MakeLine require point geometries, and I don't see
anything else that seems to be what I'm looking for. Any ideas would be
welcome.
Thanks,
Lee Hachadoorian
PhD Student in Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center
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