[postgis-users] Merge all polygons from a dataset

Mon Aug 4 05:47:53 PDT 2008

That is a little odd.  What are you using to view the data.  I know for
example QuantumGIS leaves out some geometries (particularly when they
are large geometries) unless you change some setting on it.  I noticed
this once when I loaded up a large Candadian dataset and thinking -
THATS CANADA.  Then I changed some setting and all looked fine.

With ST_Collect you probably get more geometries in your MULTIPOLYGON
than ST_Union since it won't dissolve the boundaries, but you shouldn't
be getting any geometries left out.

Hope that helps,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
danny
Sent: Monday, August 04, 2008 8:24 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Merge all polygons from a dataset

Thanks Regina.

For some reason ST_Collect doesn't give exactly the same result. For
example I'm trying to merge european countries in one shape and with
ST_Collects it omits England and other islands while ST_Union includes
them. 
Anyway, my union finished after 4 hours.
I'm joining two tables on an ST_Intersect, one of which has ~15000
features (polygons) and the other just one (a complex polygon). The
query's been running for two hours... I hope I'll see the end of it
before the end of the day! (I've set the indexes where I could and tuned
PostgreSQL)

Regards. 

On Mon, Aug 4, 2008 at 12:25 PM, Obe, Regina <robe.dnd at cityofboston.gov>
wrote:

	Try the trick documented here

http://postgis.refractions.net/documentation/manual-svn/ST_Collect.html

	On hindsight - I realized the ST_Multi call here is probably
unnecessary so you can leave that part out.

	Hope that helps,
	Regina

________________________________

	From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
danny

	Sent: Monday, August 04, 2008 5:04 AM 

	To: PostGIS Users Discussion
	Subject: Re: [postgis-users] Merge all polygons from a dataset

	I'm getting very long processing time from ST_Union (on a shape
of europe at 1/100000 it's been running since 2 hours and hasn't
finished yet; on my previous one of 1/1000000 it took ~1 hour)
	Any other alternative and quicker ways to accomplish the same
operation?
	I just executed st_buffer(st_collect(the_geom)) but still
waiting. Should it take as long? ST_Collect by itself executes fast but
it's result violates the geometry check constraint. 

	On Fri, Aug 1, 2008 at 5:31 PM, danny <whatevar89 at gmail.com>
wrote:

		Thanks Robert.

		I actually exported from ogr2ogr and it created a shape
that worked fine in ArcView.

		Best 

		On Fri, Aug 1, 2008 at 5:19 PM, Burgholzer,Robert
<rwburgholzer at deq.virginia.gov> wrote:

			Could ArcView perhaps want some columns other
than just the shape?  Perhaps you could put a numeric ID or something
else in there.

			Just a thought,

			Robert W. Burgholzer

			Surface Water Modeler

			Office of Water Supply and Planning

			Virginia Department of Environmental Quality

			rwburgholzer at deq.virginia.gov

			804-698-4405

			Open Source Modeling Tools:

			http://sourceforge.net/projects/npsource/

			-----Original Message-----
			From:
postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
danny

			Sent: Friday, August 01, 2008 9:30 AM
			To: PostGIS Users Discussion
			Subject: Re: [postgis-users] Merge all polygons
from a dataset

			Many thanks, Regina! Indeed ST_Union generated
the polygon, invisible in PgAdmin!! 

			It creates a nice layer when I open it in uDig. 

			However when I export it with pgsql2shp, the
shape is unreadable by ArcView with the errmsg: Number of shapes does
not match the number of table records.

			Here's how I created the layer:

			create table europe_contour();
			SELECT addgeometrycolumn('db, 'europe_contour',
'the_geom', 32767,'MULTIPOLYGON', 2 );
			insert into europe_contour(the_geom) values(
(select st_union(the_geom) from europe));

			Then I generate the shapefile: 

			"c:\Program
Files\PostgreSQL\8.3\bin\pgsql2shp.exe" -u postgres -P password -f
europe_contour db europe_contour

			Any suggestions perhaps on anything I've wrong?

			On Fri, Aug 1, 2008 at 1:15 PM, Obe, Regina
<robe.dnd at cityofboston.gov> wrote:

			Erik,

			How were you using ST_Union and what exactly do
you mean by no result.  no result as it looks blank?  If you are using
something like PgAdmin III - it will look blank because it exceeds the
buffer supported by PgAdmin III.

			SELECT the_geom is null 

			FROM (somequery or table) as foo

			LIMIT 1

			will tell you if its actually returning no
geometry

			Hope that helps,

			Regina

________________________________

			From:
postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
danny
			Sent: Friday, August 01, 2008 6:12 AM
			To: postgis-users at postgis.refractions.net
			Subject: [postgis-users] Merge all polygons from
a dataset

			Hello everybody,

			I've converted a shapefile of Europe to PostGIS.
I would like to merge all the countries together so that I only have one
polygon left, the whole contour of Europe. I have tried ST_Union that
gives me no result. I've tried several other functions to no avail. Does
anybody know how I should proceed?

			Many thanks,

			Erik

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