# [GRASS-dev] boundaries, areas and between

Hamish hamish_nospam at yahoo.com
Tue Feb 27 23:36:39 EST 2007

```Moritz Lennert wrote:
> Maciej Sieczka wrote:
> >
> > If there is only one, valid, area, it still is green, but describing
> > it as "Boundary (between 2 valid areas)" or "Boundary (2 areas)"
> > does not make sense.
>
> Unless you take into account that the rest of the universe is counted
> as a valid area.
>
> >  and orange "Boundary (between 1 complete
> >> area and one not good area)"; and grey "Boundary (which doesn't
> >> form an area)".
> >
> > Proposition:
> > was:                 to be:
> > Boundary (no area)   Boundary not closed
> > Boundary (1 area)    Boundary closed, invalid
> > Boundary (2 areas)   Boundary closed, valid
>
> I don't think that the boundary which has only one area next to it is
> invalid.
>
> > I have one more problem. Can you look at my drawings. Shouldn't the
> > east-most area be either all-good-green (vdig_badarea_ms.png) or
> > all-bad-orange (vdig_badarea_ms2.png), and not a mix of the two, as
> > it is?
>
> The way I understand this is that the rest of the universe is
> considered as a valid area, unless it is in "touch" with an unclosed
> boundary. At that moment the rest of the universe is not "rest of the
> universe" anymore but the potential content of this unclosed boundary.
> So, the boundaries are orange because what should be the rest of the
> universe, and thus a valid area, cannot be considered as such. If you
> close the western area, then the rest of the universe becomes a valid
> area and each boundary thus delimites two areas.
> The one boundary which is green is the only one between two areas.
>
> I hope I make any sense ;-)

Yes, you have it correct.

Hamish

```