[geos-devel] Re: TopologyException makes GEOS/JTS very difficult to employ in my production environments...

[Martin Davis]

  In case it helps, you can deal with this invalidity by running either 
UnaryUnion or buffer(0) over the invalid MultiPolygon.  When unary union 
is used, difference() then works on the results.

On 9/29/2010 9:15 AM, Martin Davis wrote:
>  I'll make the easy reply first.   8^)
>
> The invalidity is due to one component polygon overlapping another one.
>
> See the attached diagram which displays the problem (generated using 
> the Magnify Topology capability in the development version of JTS 
> TestBuilder)
>
> Also, many of the segments of this component polygon are coincident 
> with the adjacent polygon boundary segments.  This violates the rule 
> from SFS 2.1.12 that you quote, since segments contain an infinite 
> number of points.
>
> Martin
>
> On 9/29/2010 8:16 AM, G. Allegri wrote:
>>> Pre-empting Martin: your multipolygon is invalid.
>> Thanks Paul, I've learned something I didn't know... I thought that a
>> MP where two polygons touch on a vertice was valid.
>> > From SFS Specifications (par. 2.1.12):
>>
>> "The Boundaries of any 2 Polygons that are elements of a MultiPolygon
>> may not ‘cross’ and may touch
>> at only a finite number of points"
>>
>> JTS tells me that I have a self-intersection, but visually I only see
>> two vertices touchin at 1664458.5096082322,1664458.5096082322
>>
>>> However: yes, there are lots of people who do big geoprocessing with
>>> PostGIS who inevitably, in sieving millions of features through the
>>> hopper, find pairs that cause topology exceptions. Would I like that
>>> not to happen: oh yes. Do I have the time and money to fund the
>>> development to conclusively remove those cases? I do not. Let's
>>> randomly say it would take 100,000 euros to put a stake through the
>>> heart of this one and fro all and see if anyone's pain rises to that
>>> level.
>> Well, I don't have 100.000, neither my company. I will keep
>> TopologyExcpetions here and there :)
>>
>> Giovanni
>>
>>> Paul
>>>
>>> On Wed, Sep 29, 2010 at 7:15 AM, G. Allegri<giohappy at gmail.com>  wrote:
>>>> Here are the WKB representations of a polygon and a multipolygon which
>>>> cause a TopologyException for ST_Difference:
>>>>
>>>> Polygon: http://pastebin.com/rWAGMmME
>>>> Multipolygon: http://pastebin.com/sik9jZk2
>>>>
>>>>
>>>>
>>>> 2010/9/29 G. Allegri<giohappy at gmail.com>:
>>>>> Sorry for this strong title, but I would like to open a discussion on
>>>>> this topic. There are already several tickets about TopologyException
>>>>> happening in various contexts, and I now it is not an easy to solve
>>>>> problem. We, in my company, have struggled to circumvent this 
>>>>> frequent
>>>>> error, and we put some (few) money to let Sandro (strk) analyze it.
>>>>> The only solutions, at now, have been various workarounds that (not
>>>>> deterministically) work for some cases/datasets, but fail with 
>>>>> others.
>>>>> Our first "meet" with this exception was due to GeomUnion operations.
>>>>> Today we face it for ST_Difference op in PostGIS, and this time no
>>>>> workaround is working.
>>>>>
>>>>> I anticipate the critics: put the money on the table and someone 
>>>>> could
>>>>> invest time to solve it. Holy words, this is how foss should work. 
>>>>> But
>>>>> I'm not the boss of my company, and I've suggested to employ
>>>>> PostGIS->GEOS in a big job... and now I have to solve this issue, 
>>>>> with
>>>>> no extra-money.
>>>>>   If this problem cannot be solved I have to abandon PostGIS for 
>>>>> one of
>>>>> our production environments, and that would be a pity.
>>>>>
>>>>> So, my reflection is: in a few weeks we have fighted, almost daily,
>>>>> with this error. Are we the only ones to get stuck in it? Have others
>>>>> found consistent,deterministic solutions?
>>>>>
>>>>> I would be gratefull if you can share your experience and thoughts...
>>>>> Giovanni
>>>>>
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>
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022