[R-sig-Geo] contour lines on a plot

[Virgilio Gomez Rubio]

Dear Maxime,

I have not been able to reproduce your example because I lack your data
file (file.txt). However, I believe that the problem is at

> spplot(p,zcol="K1.pred",col.regions=gray(0:100/100), cuts=40, 
> sp.layout=list(pts), pretty=T)
> contour(p, add=T, nlevels=2)

You are combining lattice graphics (spplot()) and 'traditional' graphics
(contour()). A good alternative here is to pass the contour lines  to
spplot via the sp.layout parameter. You can use ContourLines2SLDF to
convert contour lines to a SpatialLinesDataFrame.

Hope this helps.

Virgilio