<br><br><div class="gmail_quote">---------- Forwarded message ----------<br>From: <b class="gmail_sendername">G. Allegri</b> <span dir="ltr"><<a href="mailto:giohappy@gmail.com">giohappy@gmail.com</a>></span><br>Date: 2011/11/24<br>
Subject: Re: [GRASS-user] Re: what are the side effects of working with topologically incorrect lines?<br>To: Markus Metz <<a href="mailto:markus.metz.giswork@googlemail.com">markus.metz.giswork@googlemail.com</a>><br>
<br><br>Hi Markus, <br>thanks for The link!<br>I haven't been able to find an official reference of G1 scheme, it was cited by the tutorials from the unviersity of Trento:<br><br><a href="http://www.ing.unitn.it/%7Ezatelli/cartografia_numerica/slides/Vettoriale_GRASS_6.4.pdf" target="_blank">http://www.ing.unitn.it/~zatelli/cartografia_numerica/slides/Vettoriale_GRASS_6.4.pdf</a><br>
<a href="http://www.ing.unitn.it/%7Ezatelli/environmental_data_management/GIS_data_modeling.pdf" target="_blank">http://www.ing.unitn.it/~zatelli/environmental_data_management/GIS_data_modeling.pdf</a><br><br>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.<br>
The SFS allows not simple geometries, like self-intersectnig linestrings. The GRASS programming manual only states:<br><br> - 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.<br>
- 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.<br>
<br>It never says if lines can self-cross. I suppose that is implicitly means they can.<br>It would be good to have a geometrical model description like the JTS Specs: <a href="http://www.vividsolutions.com/JTS/bin/JTS%20Technical%20Specs.pdf" target="_blank">http://www.vividsolutions.com/JTS/bin/JTS%20Technical%20Specs.pdf</a><br>
<br>giovanni<div class="HOEnZb"><div class="h5"><br><br><br><div class="gmail_quote">2011/11/23 Markus Metz <span dir="ltr"><<a href="mailto:markus.metz.giswork@googlemail.com" target="_blank">markus.metz.giswork@googlemail.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi Giovanni,<br>
<br>
the most "offical" and up-to-date documentation I could find is here<br>
<br>
<a href="http://grass.osgeo.org/programming7/vectorlib.html#vlibTopoManagement" target="_blank">http://grass.osgeo.org/programming7/vectorlib.html#vlibTopoManagement</a><br>
<br>
I don't know by what scheme the GRASS vector topology is inspired.<br>
What exactly is G1, do you have some reference/link for me?<br>
<br>
Ciao,<br>
<br>
Markus<br>
<div><div><br>
<br>
On Wed, Nov 23, 2011 at 10:59 AM, G. Allegri <<a href="mailto:giohappy@gmail.com" target="_blank">giohappy@gmail.com</a>> wrote:<br>
> Thanks Markus for the reply,<br>
> is there an official reference to the GRASS topological model? I mean<br>
> something more detailed then this [1].<br>
> Googling I've found that it's inspired to G1 scheme, Full Planar Topology,<br>
> and other similar definitions, but I haven't been able to find an official<br>
> reference/definition.<br>
> thanks,<br>
> giovanni<br>
> [1] <a href="http://grass.fbk.eu/grass64/manuals/html64_user/vectorintro.html" target="_blank">http://grass.fbk.eu/grass64/manuals/html64_user/vectorintro.html</a><br>
><br>
> 2011/11/23 Markus Metz <<a href="mailto:markus.metz.giswork@googlemail.com" target="_blank">markus.metz.giswork@googlemail.com</a>><br>
>><br>
>> G. Allegri wrote:<br>
>> > Probably I'm wrong: the self intersecting line doesn't produce errors<br>
>> > with<br>
>> > v.build.<br>
>> > Mmm, I thought it wasn't compliant with G1 scheme...<br>
>> > giovanni<br>
>> ><br>
>><br>
>> As far as the GRASS topological model is concerned, self-intersecting<br>
>> lines are allowed, self-intersecting boundaries are not.<br>
>><br>
>> Self-intersecting lines are ok for <a href="http://v.net" target="_blank">v.net</a> modules, e.g. to represent a<br>
>> bridge of a secondary road over a highway. There are some modules that<br>
>> do not like self-intersecting lines, e.g with v.buffer I would expect<br>
>> problems.<br>
>><br>
>> Markus M<br>
>><br>
>> > 2011/11/22 G. Allegri <<a href="mailto:giohappy@gmail.com" target="_blank">giohappy@gmail.com</a>><br>
>> >><br>
>> >> I'm studying better the topological scheme assumed by GRASS, which if<br>
>> >> I'm<br>
>> >> not wrong is G1.<br>
>> >> I'm doing some basic tests to understand the side effects of working<br>
>> >> with<br>
>> >> not topologically clean entities, like self-intersecting lines.<br>
>> >> Original line: <a href="http://bit.ly/vfIJiM" target="_blank">http://bit.ly/vfIJiM</a><br>
>> >> Cleaned up line: <a href="http://bit.ly/uHKpoT" target="_blank">http://bit.ly/uHKpoT</a><br>
>> >> The unclean line is topologically incorect, right?<br>
>> >> Where the not topological version can give us problems, (excluding the<br>
>> >> <a href="http://v.net" target="_blank">v.net</a> modules)?<br>
>> >> The question raises from the need to understand if I can bypass line<br>
>> >> cleaning for various data processing I'm going to do, mainly overlay,<br>
>> >> data<br>
>> >> extraction, profile analysis, etc.<br>
>> >> Thanks,<br>
>> >> giovanni<br>
>> ><br>
>> ><br>
>> > _______________________________________________<br>
>> > grass-user mailing list<br>
>> > <a href="mailto:grass-user@lists.osgeo.org" target="_blank">grass-user@lists.osgeo.org</a><br>
>> > <a href="http://lists.osgeo.org/mailman/listinfo/grass-user" target="_blank">http://lists.osgeo.org/mailman/listinfo/grass-user</a><br>
>> ><br>
>> ><br>
><br>
><br>
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