comparison of 2 different files with same resolution

[DM]

Hello GrADS users,

here's my problem:

I would plot differences of meteo fields produced by 2 MM5 runs sharing
EVERYTHING but the external domain extension.

Because of the same grid-spacing (both horizontal and vertical) I expected
that the simple following commands

open file1.ctl  (--> the largest domain)
set t 2
set z 15
open file2.ctl
set t 2
set z 15
d ght.1 - ght.2

would have worked.
The only result was:

Operation error:  Incompatable grids
  Dimension ranges aren't equivalent

I' ve tried to define a lat-lon box for both the domains, to define a new
variable for every file, to open e close files, to use w2gr gr2w and to
combine all these things...but I've always got the same error.

So, although the 2 files have the same resolution, should I use the
regridding function (http://opengrads.org/doc/udxt/re/) or is there a
simple way I've still not discovered?

Waiting for you replies,

Thank you

Daniele