Fwd: [Qgis-user] Table with empty wkb_geometry

[Matej Serc]

---------- Forwarded message ----------
From: Matej Serc <mailing at tam.si>
Date: 2011/7/22
Subject: Re: [Qgis-user] Table with empty wkb_geometry
To: Andreas Neumann <a.neumann at carto.net>


Hello Andreas, you were right! There are some self-intersections in those
parcels that are not returned correctly in intersections query.

Is there any method to "correct" this issue as much automatically as
possible?

Thanks,
Matej


2011/7/21 Andreas Neumann <a.neumann at carto.net>

> Hi Matej,
>
> I would assume that the geometries are not empty to begin with. Some
> software (like pgadmin) does not show you a WKB geometry if the geometry is
> too big. But this does not mean that the geometry is empty.
>
> There are methods in Postgis to see if the geometry is empty:
> http://postgis.refractions.**net/documentation/manual-1.5/**
> ST_IsEmpty.html<http://postgis.refractions.net/documentation/manual-1.5/ST_IsEmpty.html>
>
> SELECT gid FROM your_schema.your_table WHERE ST_IsEmpty(the_geom);
>
> There is also a method to see if a geometry is valid (contains errors):
> see http://postgis.refractions.**net/documentation/manual-1.5/**
> ST_IsValidReason.html<http://postgis.refractions.net/documentation/manual-1.5/ST_IsValidReason.html>
>
> SELECT gid, ST_IsValidReason(the_geom) FROM your_schema.your_table WHERE
> ST_IsValid(the_geom) = FALSE;
>
> Often, data imported from Shapefiles, contains errors, such as self
> intersections, too few points, etc. - ST_IsValidReason() helps to find the
> geometries. If you use the node tool in QGIS while in an editing session,
> such errors are marked in green (no idea why they aren't marked in red - red
> would be the color for errors in my opinion).
>
> Hope this helps.
>
> Andreas
>
>
> On Thu, 21 Jul 2011 00:33:29 +0200, Matej Serc wrote:
>
>> Hi all,
>>
>> this is just partially connected with QGIS, but since I experience
>> some behaviour which I can't understand, I will ask here.
>>
>> We have some parcel data which were imported to PostGIS DB from SHP
>> files. I have just found out that there is a small percentage of the
>> rows that have wkb_geometry field empty, but the parcel is visible in
>> the QGIS, so I can click on it and Identify results etc.
>>
>> Basically I started looking at it more deeply when using some
>> intersection functions from PostGIS that did not list some parcels
>> which should by no means be listed. Now after looking at the SELECT
>> query output in the pgAdmin, I see empty field for such parcels.
>>
>> What bothers me, is: how can these parcels be visible in QGIS? I think
>> that there was some problem when I was importing the data from SHP -
>> but no errors were shown in ogr2ogr at that time. Did anyone already
>> experienced something like this?
>>
>> Thank you very much for any ideas and pointers in advance :-)
>> BR, Matej
>>
>
> --
> --
> Andreas Neumann
> Böschacherstrasse 10A
> 8624 Grüt (Gossau ZH)
> Switzerland
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