[geos-devel] Re: TopologyException: found non-noded intersection

[Martin Davis]

Giovanni,

That's a good point. In both JTS and GEOS CascadedUnion is limited to 
operating in main memory.  (However, it may be that GEOS running under 
PostGIS is more memory-efficient, or is able to utilize a larger memory 
space).

There's not really an easy solution for working with JTS CascadedUnion 
and very large datasets.  (This also applies to using CascadedUnion in 
JEQL - although JEQL itself is not memory-bound, it can process data in 
a streaming fashion.  But the CascadedUnion algorithm is not stream-based).

Some alternatives that come to mind are:

- use 64-bit Java and lots of memory

- implement your own "GriddedUnion" in PostGIS (and maybe involving 
JTS).  I can provided details if you like.

Martin

G. Allegri wrote:
> Thanks Martin.
> I see that jts 1.11 works fine on a subset of my dataset. I have
> hundreds of thousands od polygons to merge... I have to find the way
> to read them to pass the collection to JTS without receiving
> java.lang.OutOfMemoryError (just happened!).
> What do you suggest to build the collection needed by CascadedUnion
> without havind to create a huge Collection of geometries?
>
> I will take a look at JEQL. Thanks for the hint, I didn't know it...
>
> Giovanni
>
> 2010/6/22 G. Allegri <giohappy at gmail.com>:
>   
>> I'm using Postgis 1.5.1 to compute a Union on a quite large
>> multipolygons dataset (some hundreds of thousands of features), but
>> I'm facing the following error:
>>
>> NOTICE:  TopologyException: found non-noded intersection between
>> LINESTRING (1.7318e+006 4.77959e+006, 1.7318e+006 4.77958e+006) and
>> LINESTRING (1.7318e+006 4.77958e+006, 1.73174e+006 4.77954e+006) at
>> 1.7318e+006 4.77958e+006
>>
>> ERROR:  GEOS union() threw an error!
>>
>> I know it's a known issue, but I haven't been able to find a good,
>> replicable, solution. The input geometries result valid (in simple
>> feature meaning).
>> What is causing this? I have overlapping polygons, but I thought the
>> cascaded union algorithm computed the necessary noding on the
>> linestrings.
>> I've supposed it depends on tolerances/precision model, but I don't if
>> it makes sense and how, eventually, tune it.
>>
>> Thanks a lot,
>> Giovanni
>>
>>     
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>   

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