[GRASSLIST:8812] r3.in.ascii

[s99171059 at tuks.co.za]

Dear list,

I have a raster array which represents cell phone signal strengths, and would
like to import it into grass 6.  Looks like r3.in.ascii might be what I need to
import this raster array, as I would like to display the signal strengths in
different colours.  The help documentation on r3.in.ascii seems to be quite
concise.  The file nees a header: 

north: floating point
south: floating point
east: floating point
west: floating point
top: floating point
bottom: floating point
rows: integer
cols: integer
levels: integer

I'm not sure what the top, bottom and levels values should be.  Can someone
pleas elaborate on this?  This is what I found in the help files:

This header is followed by the cell values in floating point format organized in
rows with constant col and level coordinate. The rows are organized by constant
level coordinate. Individual cell values are separated by space or CR.

Regards,
Kobus