Problem with SpatialPolygons containing a hole

On Thu, 29 Jan 2009, Hans-J?rg 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.

Roger

Cheers,
--Hans

--
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