[R-sig-Geo] Overlaying rasters in spplot

[Chris Fowler]

I am still having trouble with some raster overlays.
My trouble now is a result of my not really understanding how layers
and sp.layout work in the spplot function

I go through a whole mess of steps to get two rasters of identical
extent and dimension (One is a google map tile, the other my own data)
Plotting them using the default plot() method within raster, however
doesn't allow me to have the color palette and breaks I want, so at
the suggestion of one of the threads on this list I have converted my
raster objects to SpatialGridDataFrame objects.

I can plot these objects and get exactly what I want using the following code:
  layer1<-spplot(raster1,"values",at=brks1,col.regions=pal1,colorkey=FALSE)
  layer1
  layer2<-spplot(raster2, "values", at=brks2,col.regions=pal2)
  layer2

from reading the documentation on spplot() from sp package I should be
able to use the sp.layout attribute to map the second of these layers
with a transparency of say .5, but I have not been able to get there.

I am hoping that this is enough of a syntax question that the experts
here can help without a full worked example--my apologies if this is
not the case.

Thanks in advance,

Chris

Christopher S. Fowler PhD.
206.920.1686
http://csfowler.com