[GRASSLIST:2110] re: r.mapcalc

[Eric G. Miller]

On Tue, Jul 17, 2001 at 12:04:13PM +0800, Rohan Sadler wrote:
> Dear GrassList,
> 
> I have just started on Grass last week and am using ver 5.0beta11 on 
> redhat 7.1.
> 
> r.mapcalc does not work for me so that any expression such as
> 
> test3 = test2 - test1                    (with test2 set as MASK)
> test3 = if(test2 != test1, 1, 0)
> 
> returns just the intersection of test2 and test1,
> and not what I want - which is that part of test2 that does not include 
> test1 (i.e. test2 minus the intersection).
> 
> If anyone has any ideas before I reinstall GRASS please forward them.

Nothing wrong with r.mapcalc by your description. 

1. Make sure to set the region appropriately "g.region rast=test1,test2"
2. Read the part of the manual page dealing with NULLs and the various
   related functions/tests.
3. Remove the MASK (otherwise you'll get nothing).
4. Try "$ echo 'test3 = if(test1, null(), test2)' | r.mapcalc"

   Which translates to:

      if "test1" is not null, output null
      otherwise output test2.

Alternatively, if I understand correctly, you could set the MASK using
test1 and just "echo 'test3 = test2' | r.mapcalc".  I think that should
work to get all of test2 that doesn't intersect with test1.

-- 
Eric G. Miller <egm2 at jps.net>