[GRASS-user] Re: what are the side effects of working with topologically incorrect lines?

Markus Metz markus.metz.giswork at googlemail.com
Wed Nov 23 04:14:04 EST 2011

G. Allegri wrote:
> Probably I'm wrong: the self intersecting line doesn't produce errors with
> v.build.
> Mmm, I thought it wasn't compliant with G1 scheme...
> giovanni

As far as the GRASS topological model is concerned, self-intersecting
lines are allowed, self-intersecting boundaries are not.

Self-intersecting lines are ok for v.net modules, e.g. to represent a
bridge of a secondary road over a highway. There are some modules that
do not like self-intersecting lines, e.g with v.buffer I would expect

Markus M

> 2011/11/22 G. Allegri <giohappy at gmail.com>
>> I'm studying better the topological scheme assumed by GRASS, which if I'm
>> not wrong is G1.
>> I'm doing some basic tests to understand the side effects of working with
>> not topologically clean entities, like self-intersecting lines.
>> Original line: http://bit.ly/vfIJiM
>> Cleaned up line: http://bit.ly/uHKpoT
>> The unclean line is topologically incorect, right?
>> Where the not topological version can give us problems, (excluding the
>> v.net modules)?
>> The question raises from the need to understand if I can bypass line
>> cleaning for various data processing I'm going to do, mainly overlay, data
>> extraction, profile analysis, etc.
>> Thanks,
>> giovanni
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