[R-sig-Geo] AsciiGrid - draw boundary

[Muhammad Rahiz]

Dear all,

I'm working with data in the form of an ascii grid (British Ordnance 
survey grid). I've plotted the data using image.plot with success and 
would like to draw a boundary around the region so that values which are 
zero and the white plotting region can be easily distinguished.

Has anyone any experience with this? Appreciate some guidance...

Thanks       

Muhammad