[GRASS5] Re: Contents of grass5 digest, Vol 1 #501, islands

[Scott O'Donnell]

Islands indicate either, an area completely surrounded by a different 
area, or a 'hole' (a null attribute area) in an otherwise uniform field.   
The 'hole' is really just a special case  island.  

Neverleless, I think that islands should be managed better than assuming 
that they have no significance.  I believe that shapefiles previously 
ignored islands, but now allow nested polygons with unique attributes. 

Perhaps if the areas were rendered in inverse order of size the smaller
areas (islands) would end up painted on top of the larger areas and not
be lost in the visualization of the data.

Scott O'Donnell

> 
> Message: 1
> From: Radim Blazek <blazek at itc.it>
> To: grass5 at grass.itc.it
> Subject: Re: [GRASS5] Re: [GRASS-CVS] eric: grass/src/mapdev/v.out.shape extract.c,1.8,1.9
> Date: Fri, 4 Oct 2002 13:09:01 +0200
> 
> On Friday 04 October 2002 09:58 am, Markus Neteler wrote:
> > Hi Eric,
> >
> > thanks for fixing the memory corruption bug: now commands like
> >
> > v.out.shape areamap type=area
> >
> > do no longer crash. But... :-)
> > If the map contains islands, they are still filled by the surrounding
> > area.
> >
> > If ever possible, maybe somebody could look into that (or I
> > am missing something, probably islands in SHAPE are a problem?
> > Note that I checked the SHAPE file with Arcexplorer for Linux,
> > maybe the problem is there).
> 
> As each island in GRASS is also area, it is exported
> as area to shapefile with CAT_ID = 0. Such shapefile is then displayed
> with filled island (not island but area in the island) if all areas
> have the same number. If a different colour is assigned to each CAT_ID,
> the area in the island has a different colour.
> We need a new flag to disable export of not labeled areas.
> 
> Radim
> 
> 
> Message: 7
> Date: Fri, 4 Oct 2002 08:35:16 -0700
> From: "Eric G. Miller" <egm2 at jps.net>
> To: grass5 at grass.itc.it
> Subject: Re: [GRASS5] Re: [GRASS-CVS] eric: grass/src/mapdev/v.out.shape extract.c,1.8,1.9
> 
> On Fri, Oct 04, 2002 at 09:58:52AM +0200, Markus Neteler wrote:
> > Hi Eric,
> >
> > thanks for fixing the memory corruption bug: now commands like
> >
> > v.out.shape areamap type=area
> >
> > do no longer crash. But... :-)
> > If the map contains islands, they are still filled by the surrounding
> > area.
> >
> > If ever possible, maybe somebody could look into that (or I
> > am missing something, probably islands in SHAPE are a problem?
> > Note that I checked the SHAPE file with Arcexplorer for Linux,
> > maybe the problem is there).
> 
> Islands are presumably holes and need to have their direction reversed.
> Shapefiles use a winding rule to determine interior/exterior boundaries.
> 
> --
> begin 664 .signature
> M<F5L;&E-("Y'(&-I<D4@/G1E;BYS<&I`,FUG93P)"`@("`@("`@("`@("`@(
> M"`@("`@("`@("`@("`@("`A%<FEC($<N($UI;&QE<B`\96=M,D!J<',N;F5T
> "/@H`
> `
> end
> 
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-- 
Reality is that which, when you stop believing         |       Scott O'Donnell
in it, doesn't go away.                                | odonnell at fsl.noaa.gov
                           -- Philip K. Dick           |          303·497·6562