I am plotting a krigged values of irrdiance with the following command.

spplot(p1, zcol="insol.pred", col.regions=terrain.colors(30), cuts=30,
sp.layout=list(pts1), contour=TRUE, labels=TRUE, pretty=TRUE, col='brown',
main=title1)

I want to overlay California (for that matter any future state for which I
do similar work) on the plot

How am I going to do that. Where am I going to get the map information and
which command is used. I was trying gadm and there was no success for me. I
think I am missing something very basic.

Thank you for your time.

Best,

Aleu

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