PostGIS users

Obe, Regina | 1 Aug 2008 13:15

RE: Merge all polygons from a dataset

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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danny | 1 Aug 2008 15:29

Re: 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

The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. It is intended solely for the addressee. If you received this in error, please contact the sender and delete the material from any computer.

Help make the earth a greener place. If at all possible resist printing this email and join us in saving paper.

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_______________________________________________
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