# AW: [GRASS-user] how to select island polygons with grass

Christian.Strobl at dlr.de Christian.Strobl at dlr.de
Wed Jun 25 11:17:01 EDT 2008

```hi markus,

thanks for your quick answer, but i made a mistake. the pseudo code i wrote was to select dangles, the selection criteria for an island is a polygon whose boundary consists only of one line

for example

--------------x--------------
| A            |              |
|    -----     |              |
|   |     |    |b             |c
|   | B   |d   |     C        |
|   |     |    |              |
|    -----x    |              |
|              |              |
--------------x--------------
a

A,B,C = polygons
a,b,c,d = lines
x= nodes

a normal polygon (A,C) has at least two boundaries,
an island polygon (B) has usually only one boundary (d)

do you also know a solution for this query?

thanks and excuse the wrong description
christian

> -----Ursprüngliche Nachricht-----
> Von: neteler.osgeo at gmail.com [mailto:neteler.osgeo at gmail.com]
> Im Auftrag von Markus Neteler
> Gesendet: Mittwoch, 25. Juni 2008 16:59
> An: Strobl, Christian
> Cc: grass-user at lists.osgeo.org
> Betreff: Re: [GRASS-user] how to select island polygons with grass
>
> On Wed, Jun 25, 2008 at 3:46 PM,  <Christian.Strobl at dlr.de> wrote:
> >
> > hi all,
> > principally an easy question but i have spent a few hours
> > and found nothing. is there a possibilty with grass to
> select island
> > polygons from a grass vector map, for example with arcinfo
> it was very
> > easy
> >
> > Pseudocode: select Polygons from polygonLayer where leftPolygon =
> > rightPolygon
>
> With v.to.db you can select left and right sides.
> Usage example:
> https://stat.ethz.ch/pipermail/r-sig-geo/2005-October/000616.html
>
> > p.s. exist any possibilities to select geometries with other
> > topological criteria, for example dangle lines. i know that is
> > possible to delete them with clean but i am looking for a
> possibility
> > only to select them
>
> v.edit can select:
> http://grass.osgeo.org/grass64/manuals/html64_user/v.edit.html
>
> In the new digitizer in wxpython there should be such
> functionality to catch dangles.
>
> Hope this helps a bit
> Markus
>
> --
> Open Source Geospatial Foundation
> http://www.osgeo.org/
> http://www.grassbook.org/
>
```