[postgis-users] noding problem
Chris Hermansen
chris.hermansen at timberline.ca
Mon Oct 29 10:54:32 PDT 2007
Given my headspace on cleaning up the crummy data I recently received,
I'm not at all disappointed to receive a warning that I have some
two-point, zero-length lines, and it certainly seems harmless to remove
such creatures from my linework.
When I first saw the message I thought "oh these should be LINEs, not
LINESTRINGs"... old Microstation synapses firing there :-)
Martin Davis wrote:
> Chris,
>
> When I originally determined the semantics for JTS for some reason I
> thought that the OGC-SFS implied that two-point linestrings with both
> points identical were invalid. Upon re-reading the spec I can't see
> anything that actually states this. So perhaps this rule is overly
> restrictive.
>
> At least the situation is easy to check for and work around.
>
> Martin
>
>
> Chris Hermansen wrote:
>> Martin, everyone.
>>
>> Just a coda to this noding problem.
>>
>> As Martin pointed out, there is some exceedingly small nasty stuff in my
>> linework. In order to try to get rid of it, I tried both
>> st_simplify(the_geom,0.01) and st_snapToGrid(the_geom,0.01).
>>
>> Both seemed to remove the problem, based on running st_union on the
>> results.
>>
>> st_simplify gave me back about half as many linestrings as
>> st_snapToGrid.
>>
>> The other thing that st_simplify gave me back was several linestrings,
>> which st_isValid() subsequently rejected with messages like:
>>
>> NOTICE: Too few points in geometry component at or near point
>> 1.2128e+06 906134
>>
>> These seem to be two-point, zero-length linestrings, eg:
>>
>> quesnel=# select numPoints(the_geom),length(the_geom) from step2b where
>> not isValid(the_geom);
>>
>> numpoints | length
>> -----------+--------
>> 2 | 0
>> 2 | 0
>> 2 | 0
>> ...
>>
>> Since the length of these linestrings is zero (or pretty close to it) I
>> decided to toss them in this case.
>>
>> But I'm a bit puzzled as to the error messages from st_isValid(). I see
>> OGC docs that refer to "two point linestrings" so I'm not really sure
>> what st_isValid() is complaining about.
>>
>> Does anyone have any ideas?
>>
>> Martin Davis wrote:
>>
>>> Chris,
>>>
>>> There's a big difference between "looks ok" and "IS ok". I suspect
>>> there are very tiny discrepancies between lines which appear to be
>>> fully coincident at normal zoom levels. This would be a typical
>>> result from intersecting polygons which are "adjacent" but in fact not
>>> fully noded together.
>>>
>>> I can envisage some kind of data cleaning operation which would "snap"
>>> and "crack" nodes and line segments in a set of geometries according
>>> to a specified tolerance. Would that solve your problem? Currently
>>> there's nothing like this in PostGIS. It would be an interesting
>>> project to design it...
>>>
>>> Chris Hermansen wrote:
>>>
>>>> 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!
>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> postgis-users mailing list
>>>>> postgis-users at postgis.refractions.net
>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>>
>>>>
>>
>>
>>
>
--
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.
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