[R-sig-Geo] Manipulating SpatialPolygonsDataFrames

[Rob Robinson]

Dear All,
 I am trying to combine various layers of data into a single image. The last
layer is a sea mask to blank out some extraneous kernel areas. I have a map
(read in from an ESRI shapefile using readShapePoly) as a
SpatialPolygonsDataFrame, which obviously has all the countries as
'foreground', does anyone know of an easy to invert this, so the sea becomes
forground? I can see in principle you might be able to do something with the
RingDir slot (to turn polygons into holes and vice versa), but can't figure
out how to access it to try.
 Any hints gratefully received!
Cheers
rob

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Dr Rob Robinson, Senior Population Biologist
British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU
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