[R-sig-Geo] contourline as border between cells containing numbers and those containing NA of ASCII grid

[Jan Schwanbeck]

Hi R spatial experts,

 From an ASCII grid I have made a couple of filled.contour - plots.

All cells outside the area of interest are set to NA. The cells inside
the area of investigation are numbers(real).

In my plots - both NA's and values smaller than 0.5 are "transparent".

To show the border of the area of investigation I would like to generate
a contourline or something similar in between cells containing numbers and  
those containing NA.

Is there any possibile way to do that?

If not: -> How could I overlay a filled.contour-plot with a shape file  
(the border of the area)?

Thanks a lot.

Jan Schwanbeck