[GRASSLIST:10271] how to determine percent of coverage

[Ed Davison]

I have a radar map that I am loading into a lat/long mapset.  I have
taken the original input and converted all cells of the raster to null
where there is no real radar info.  So, what I am trying to do is to
take an area, say a county, and run an analysis to determine what
percent of the area has rain (any data other than null).  I do not want
to sum all data but to get a percent of area with any value other than
null.

I think that I probably need to somehow get the area into a raster layer
so it can be used as a mask (?) for anaylsis purposes.  Is that right?

Then I will probably need to use something like r.mapcalc but cannot
figure out how to do this, even after reading through the man page
several times.

Can anyone give me any pointers on this?

Ed Davison