[R-sig-Geo] spplot vs sp.polygon - Problem when drawing polygon with holes/islands overlay

[Maarten Plieger]

Hi all,

I am having a (known) problem with plotting polygons with islands and 
holes. I would like to visually mask my raster data with a polygon 
overlay. The problem is that my polygon data has islands/holes, and 
these are not drawn correctly when used as an overlay with the spplot 
panel function.

When I use spplot directly to plot my polygons there is no problem:
spplot(countryOverlay [which(countryOverlay 
$FID_Countr==0),],"CNTRYNAME",   ylim=c(300000,624000),    
xlim=c(0,285000),col.regions=c("transparent","transparent","transparent","blue","transparent")  
)
(see spplot.png)

Now I would like to use the spplot panel function to draw the same data 
as an overlay:
spplot(countryOverlay,"CNTRYNAME" ,ylim=c(300000,624000),    
xlim=c(0,285000)  , 
   panel = function(...) {
    sp.polygons( countryOverlay [which(countryOverlay $FID_Countr==0),], 
fill = "blue",col = "black")
    }
)
But now, the islands are gone! (see sp.polygons.png)
(The example above is just to demonstrate this problem/feature, it does 
not draw my gridded data.)

Is it possible to draw the polygons as an overlay, and preserve the 
islands? Maybe use another function?
I see that sp.polygon uses grid::polygonGrob, and is documented that 
this function does not preserve the holes.
I find it strange that spplot itself can draw the dataset correctly, so 
it must be possible to draw the polygon correctly as an overlay... or not?

If people are interested, I have a small R script available with some 
test data to demonstrate the problem.

Best Regards,
Maarten Plieger

-- 
Maarten Plieger
KNMI, R&D Information and Observation Technology, De Bilt
(t) +31 30 2206330

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