[postgis-users] noding problem

[Chris Hermansen]

Hi Martin, everyone;

The polygons were converted to linestrings in the previous step, and
I've checked...

Martin Davis wrote:
> Did you convert your polygons to linestrings?  If not, then you are
> running the union operation on polygonal geometry.  This is the usual
> source of topology errors (which are caused by robustness & precision
> issues in the underlying linework).
>
> Chris Hermansen wrote:
>> Hi folks;
>>
>> I posted something a few days back on a noding problem I'm having, and
>> received no answer.  At this point I have more details and I think there
>> is something seriously wrong.
>>
>> This is Postgis 1.3.1 and GEOS 3.0.0rc4.
>>
>> The Postgis Wiki has an example of putting two somewhat overlapping
>> polygon networks together:
>>
>> http://www.postgis.org/support/wiki/index.php?ExamplesOverlayTables
>>
>> Basically, two tables of polygons are converted into linestrings and put
>> in a third table.
>>
>> Then, the table of linestrings is noded; in the Wiki, the code given is:
>>
>> CREATE TEMP TABLE noded_lines AS
>>    SELECT St_Union(the_geom) AS the_geom
>>    FROM all_lines;
>>
>> Right, my problem!  I have two tables with multi-polygons.  I convert
>> them to polygons.  I use isValid() on their geometries to make sure that
>> they are in good shape.
>>
>> Then!  I run the "noding query" - which I might add takes a
>> loooooooooooooooong time - and in the end I see:
>>
>> dustin=# create temp table step2 as
>> dustin-# select st_union(geom) as geom from step1;
>> NOTICE:  TopologyException: found non-noded intersection between
>> 1.40568e+06 507100, 1.40568e+06 507100 and 1.40568e+06 507100,
>> 1.40568e+06 507100 1.40568e+06 507100
>> ERROR:  GEOS union() threw an error!
>>
>> OK, now to me this doesn't seem at all fair!  If the st_union() command
>> is supposed to node the linework then WHY is complaining about non-noded
>> intersections?
>>
>> Thanks in advance for any light you can shed on this!
>>
>>   
>


-- 
Regards,

Chris Hermansen · mailto:clh at timberline.ca
tel:+1.604.714.2878 · fax:+1.604.733.0631
Timberline Natural Resource Group · http://www.timberline.ca
401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5

C'est ma façon de parler.