[postgis-users] noding problem

[Chris Hermansen]

I have isolated my problem with noding but I still don't know what
causes it, nor how to fix it.

By a succession of selections through smaller and smaller windows, I
have two tables; one with  38 linestrings, one with 25.  The 38
linestring table causes the st_union() to barf; the 25 linestring table 
passes through st_union() just fine.

When I zoom right in on the offending point reported by st_union(), I
don't see anything suspicious looking.  The linework in that location
appears to result from two polygons adjacent to each other, so it looks
like "one line" in the data viewer, from both data sets.

Given that the linework "looks" ok, is there a procedure for "forcing"
it to be OK, kind of like the buffer(0) trick for polygons, that could
be used to clean it up prior to st_union()?

Thanks in advance!

Kevin Neufeld wrote:
> Can you isolate the geometry that is causing the error?  If geos is
> broken, it would be nice to have a test case.
> IE. Try noding half the lines at a time, then try noding half of a
> half, etc...
>
> -- Kevin
>
> 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!
>>
>>   
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-- 
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.