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

[G. Allegri]

---------- Forwarded message ----------
From: G. Allegri <giohappy at gmail.com>
Date: 2011/11/24
Subject: Re: [GRASS-user] Re: what are the side effects of working with
topologically incorrect lines?
To: Markus Metz <markus.metz.giswork at googlemail.com>


Hi Markus,
thanks for The link!
I haven't been able to find an official reference of G1 scheme, it was
cited by the tutorials from the unviersity of Trento:

http://www.ing.unitn.it/~zatelli/cartografia_numerica/slides/Vettoriale_GRASS_6.4.pdf
http://www.ing.unitn.it/~zatelli/environmental_data_management/GIS_data_modeling.pdf

Anyway, reading the link you kindly provided me, it seems that GRASS uses a
geometrical model similar to the Simple Feature Spec (from OGC) with the
only topological constraints set on boundaries.
The SFS allows not simple geometries, like self-intersectnig linestrings.
The GRASS programming manual only states:

 - Boundaries should not cross each other (i.e., boundaries which would
cross must be split at their intersection to form distict boundaries).
*Lines can cross each other*, e.g. bridges over rivers.
 - Lines and boundaries share nodes only if their endpoints are identical.
*Lines or boundaries canbe forced to share a common node by snapping them
together*. This is particulary important since nodes are not represented in
the coor file, but only implicitly as endpoints of lines and boundaries.

It never says if lines can self-cross. I suppose that is implicitly means
they can.
It would be good to have a geometrical model description like the JTS
Specs: http://www.vividsolutions.com/JTS/bin/JTS%20Technical%20Specs.pdf

giovanni



2011/11/23 Markus Metz <markus.metz.giswork at googlemail.com>

> Hi Giovanni,
>
> the most "offical" and up-to-date documentation I could find is here
>
> http://grass.osgeo.org/programming7/vectorlib.html#vlibTopoManagement
>
> I don't know by what scheme the GRASS vector topology is inspired.
> What exactly is G1, do you have some reference/link for me?
>
> Ciao,
>
> Markus
>
>
> On Wed, Nov 23, 2011 at 10:59 AM, G. Allegri <giohappy at gmail.com> wrote:
> > Thanks Markus for the reply,
> > is there an official reference to the GRASS topological model? I mean
> > something more detailed then this [1].
> > Googling I've found that it's inspired to G1 scheme, Full Planar
> Topology,
> > and other similar definitions, but I haven't been able to find an
> official
> > reference/definition.
> > thanks,
> > giovanni
> > [1]  http://grass.fbk.eu/grass64/manuals/html64_user/vectorintro.html
> >
> > 2011/11/23 Markus Metz <markus.metz.giswork at googlemail.com>
> >>
> >> 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
> >> problems.
> >>
> >> 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
> >> >
> >> >
> >> > _______________________________________________
> >> > grass-user mailing list
> >> > grass-user at lists.osgeo.org
> >> > http://lists.osgeo.org/mailman/listinfo/grass-user
> >> >
> >> >
> >
> >
>
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