[R-sig-Geo] background colour when add=T

[Greg Snow]

One possibility would be to define a spatial polygon that represents
the entire plotting area, then use the gDifference function from the
rgeos package to create a map that excludes the land areas (or
whatever you don't want to cover) and add the resulting polygon(s).

On Thu, Sep 13, 2012 at 8:04 AM, Rob Robinson <rob.robinson at bto.org> wrote:
> Dear All,
>  I am trying to plot some abundance density maps for different species
> of birds which may be either marine or terrestrial. Having created the
> density kernel (using kde from package ks), I plot() a background map
> (actually a SpatialPolygonsDataFrame) and I can colour the land and
> sea differently using col= and bg= options respectively, I can then
> overlay my kernel using filled.contour. What I would now like to do is
> mask the inappropriate part of the map. For marine birds this is easy,
> simply replot the map using add=T and col= to 'blank' out the part of
> the kernel over land. I thought the same would be true for landbirds,
> simply use bg= to mask out the sea. Unfortunately when you have add=T
> plot (not unreasonably, I suppose) doesn't seem to honour the bg=
> argument, it just (I presume) sets it to 'transparent'. Does anyone
> know if there is a way to subvert this? A posting from a couple of
> years ago suggests probably not, but I wondered if anything had
> changed in the interim?
> many thanks in advance for any insights...
> cheers
> rob
>
> **************  Find out about Britain's birds - www.bto.org/birdfacts
>  **************
>
> Dr Rob Robinson, Principal Ecologist
> British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU
> Ph: +44 (0)1842 750050     E: rob.robinson at bto.org
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>
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-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com