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

[G. Allegri]

Hi Martin,

> See the attached diagram which displays the problem (generated using the
> Magnify Topology capability in the development version of JTS TestBuilder)

the Magnify is very nice and useful! I see the problem now...
I will checkout the trunk.

> 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.

I interpreted it the wrong way. In fact I didn't understand what
finite stood for. It's not menat as the number of vertices but the
geometrical interpretation of a line mad by an inifinite number of
points.
Thx

Giovanni

>
> 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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>
> --
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
>
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