[GRASSLIST:9823] Re: v.extract area with holes but without islands

[Roger Bivand]

On Sat, 14 Jan 2006, ivan marchesini wrote:

> Dear Roger..
> it could be I'm wrong but I think that island are automatically present
> when you have a hole inside an area poligon...
> grass dedects the island when it build topology.. it means that there is
> an hole inside the poligon but you can simply verify that there is not a
> centroid inside the island.... and this is correct because the island is
> not an area (it has not attributes....)
> instead the poligon has a centroid...

Yes, I follow you so far.

> 
> if you try to do the spatial sampling you will see that only points
> falling in the area will be sampled, instead those falling in the island
> (the hole) will be discared...
> 

But not if the vector is written out (for example as a shapefile of
type=area) and read into other software, in my case into R. But with 
subsetting in R, it's easy to fix there, I just need to remember to find 
the shape with the holes.

> I hope this help...
> 

Thanks,

Roger

> ivan
> 
> 
> 
> 
> 
> Il giorno ven, 13/01/2006 alle 12.06 +0100, Roger Bivand ha scritto:
> > move the islands from the vector representation? The 
> > output is to be used for spatial sampling, so sample points falling in
> > the 
> > holes should be discarded. I think v.build is assuming that holes
> > should 
> > be filled with islands in each case.
> 

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no