[postgis-users] SRID confusion

[Paragon Corporation]

Brent,

Which version of PostGIS are you using?  I'm guessing an older version.

 If I do this

SELECT AsText('SRID=4326;POLYGON((1 2 3, 4 5 6, 7 8 9, 1 2 3))')

I get this

"POLYGON((1 2,4 5,7 8,1 2))"

Which observe has no SRID information.

If I do this
SELECT AsEWKT('SRID=4326;POLYGON((1 2 3, 4 5 6, 7 8 9, 1 2 3))')

I get
"SRID=4326;POLYGON((1 2 3,4 5 6,7 8 9,1 2 3))"

(which observe maintains the SRID info)


So maybe an earlier version included an SRID which would obviously be 0 or
-1 since it basically throws that information away so is for all intents and
purposes is forced to unknown now.

So I guess my only suggestion - stay away from AsText if you care about SRID

And Use ST_AsEWKT or AsEWKT instead and ST_GeomFromEWKT or GeomFromEWKT.

Hope that helps,
Regina




-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
pcreso at pcreso.com
Sent: Thursday, October 30, 2008 5:37 PM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] SRID confusion

Hi Regina...

To add to my confusion:

I'm using AsText, as in the original example below.

If I try to open this multipolygon with QGIS (v1 pre or v0.11) it crashes.

However:

If I do the following in a shell script:

GEOM=`psql -d $DB -Atc "select astext(hole) from hole_multi;"` GEOM=`echo
"$GEOM" | sed 's/SRID=0;//g'` psql -d $DB -c "update hole_multi set
hole=geometryfromtext('$GEOM',4326);"

ie: remove any internal reference to the SRID & update the geomtery value
from the revised text string, then everything works perfectly.

Is this a bug in how geomunion() write the geometry? Does it include the
constituent SRID's when it shouldn't?


Thanks

   Brent


--- On Fri, 10/31/08, Paragon Corporation <lr at pcorp.us> wrote:

> From: Paragon Corporation <lr at pcorp.us>
> Subject: RE: [postgis-users] SRID confusion
> To: "'PostGIS Users Discussion'" 
> <postgis-users at postgis.refractions.net>
> Date: Friday, October 31, 2008, 10:14 AM Brent,
> 
> Are you using AsText or ST_AsEWKT.
> 
> AsText to my knowledges doesn't output SRID so it would always be 0.
> 
> Hope that helps,
> Regina
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of 
> pcreso at pcreso.com
> Sent: Thursday, October 30, 2008 4:53 PM
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] SRID confusion
> 
> Hi guys,
> 
> I'm not sure what is going on here:
> 
> I am working with some strata boundaries for a fisheries survey, 
> mostly depth based strata. SRID=4326
> 
> I also have some polygons of areas of untrawlable seabed & want to 
> generate these cropped by stratum. SRID=4326
> 
> Aas a step in this process I create a new table with a single 
> multipolygon comprising the geomunion of all the hole polygons.
> SRID=4326
> 
> insert into hole_multi
>                 select 1, geomunion(geom) from holes;
> 
> 
> If I run a query on this table:
> 
> select srid(hole), astext(hole) from hole_multi;
> 
> I get:
> 
>  4326 | MULTIPOLYGON(SRID=0;((55.78 ....
> 
> 
> So the SRID of the column is 4326, but each constituent polygon of the 
> multipolygon, despite coming from a 4326 dataset, seems to be set 
> internally to SRID=0.
> 
> The result is the same with:
>  insert into hole_multi
>                 select 1, setsrid(geomunion(geom),4326) from holes;
> 
> 
> Any ideas?
> 
> 
> Thanks,
> 
>    Brent Wood
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