[R-sig-Geo] Problem with SpatialPolygons containing a hole

[Kjetil Halvorsen]

On Thu, Jan 29, 2009 at 10:51, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Thu, 29 Jan 2009, Hans-Jörg Bibiko wrote:
>
>>
>> On 29.01.2009, at 13:08, Roger Bivand wrote:
>>
>>> Set the pbg= argument to set the "bg" (background) to something other
>>> than "transparent" - if it is transparent, the previously plotted larger and
>>> encompassing polygon is still visible. I'm afraid that the documentation,
>>> which should be in ?"SpatialPolygons-class" where the plot method is
>>> described, is in the maptools package, in ?plot.polylist - this will be
>>> fixed.
>>
>> Thanks.
>>
>> plot(pseSP,col='red', pbg='white')
>>
>> works. But this also implies that:
>> - one is not able to create a transparent hole(?)
>
> No. By assumption, a hole is smaller than the enclosing polygon surrounding
> it. Both are "Polygon" objects in a "Polygons" object, and these are plotted
> from largest to smallest. So if a hole is transparent, you will see the
> polygon lying underneath and around it, so won't see the hole. "Polygon"
> objects are simply rings of coordinates, not topological structures.
>
>> - and one has to care about the correct plotting order or to set the
>> correct pbg if that 'hole' belongs to another polygon
>
> By assumption, "Polygon" objects that are holes should only be within
> another "Polygon" object belonging to the same "Polygons" object. It is
> possible that data in the wild do not follow this structure, though it is
> hard to think of a real case of a hole (say lake) belonging to one county,
> but lying in a different county.

That surely goes in Bolivia. Everything goes in Bolivia.

Kjetil


>
> Roger
>
>>
>> Cheers,
>> --Hans
>>
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>
> --
> 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
>
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>



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