[GRASS-dev] RE : vlib api: is area an isle?

Robert Lagacé Robert.Lagace at fsaa.ulaval.ca
Sun Mar 24 10:49:52 PDT 2013

I do not know inside GRASS. But from analytical geometry, if you number not clock wise or conter clock wise, 
in case, the Area is positive and in the other it is negative (but I do not remember wich one). You need to look 
how it is implemented in GRASS. So, a hole, if correctly marked should a negative area.

Robert Lagacé

De : grass-dev-bounces at lists.osgeo.org [grass-dev-bounces at lists.osgeo.org] de la part de Martin Landa [landa.martin at gmail.com]
Date d'envoi : 23 mars 2013 14:01
À : GRASS developers list
Objet : [GRASS-dev] vlib api: is area an isle?


I am trying to find a way how to determine if an area forms an isle
using vlib API? Eg.

$ v.buffer in=roadsmajor out=rb dist=1000


17 areas (1 area with category + 16 areas (without categories) forms isles)
17 isles (one "universal isle" + 16 "holes" in area)

Is there any way how to determine from topology that given area forms
an isle. Such area has no category, but it's a weak precondition. We
can have areas with no category (no centroid defined for such area)
which do not form isles ("hole in polygon" from simple feature

I would like to go through all areas and skip the areas which form isles.

for (area = 1; area <= nareas; area) {
     if (Vect_is_area_isle(Map, area)

Thanks for any pointers (I just cannot find it), Martin

Martin Landa <landa.martin gmail.com> * http://geo.fsv.cvut.cz/~landa
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