I made typo errors, so drop th previous message. I do not know inside GRASS. But from analytical geometry, if you number nodes clock wise or conter clock wise, the Area is positive in one case and it is negative in the other (but I do not remember wich one). You need to look how it is implemented in GRASS. So, a hole, if correctly marked should give a negative area.
Robert Lagacé ________________________________________ De : [email protected] [[email protected]] de la part de Martin Landa [[email protected]] Date d'envoi : 23 mars 2013 14:01 À : GRASS developers list Objet : [GRASS-dev] vlib api: is area an isle? Hi, 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 produces 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 terminology). 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) continue; } Thanks for any pointers (I just cannot find it), Martin -- Martin Landa <landa.martin gmail.com> * http://geo.fsv.cvut.cz/~landa _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
