r.mapcalc and NULL

Michael Schulz schulzm at perm.geologie.uni-freiburg.de
Wed Sep 15 11:06:02 EDT 1999

Hello Agus !

There are two PS-Docs on the documentation page in Hannover, concerning 
map algebra with r.mapcalc. These do only cover the old NULL values, not 
the GRASS5 NULL and NO DATA values.

If you want r.mapcalc to create a raster map with only the NULL values of 
an existing map, the <if> statement in r.mapcalc gives you several 
options, how to be used (Right now, i do not know th eexactly syntax):

new_map = if (exist_map) : where exist_map not NULL, new_map gets 1

new_map = if (new_cat,exist_map) :  -- " --        , new_map gets new_cat

there are more options, i do not remember them by heart. If you want to 
use a logical not use the ^ sign (use this for your example: 
"new_map=if(^ exist_map)", this should work).

Hope this helps, Greetings



Michael Schulz

Christoph-Mang-Str.5       Geologisches Institut
79100 Freiburg             Universitaet Freiburg 
                           Albertstr. 23b
                           79104 Freiburg  

On Wed, 15 Sep 1999, Agustin Lobo wrote:

> How can I represent NULL values in r.mapcalc?
> I mean, doing something like this:
>  r.mapcalc "test3 = if(test == NULL)"
> Is there any doc explaining the use of the NULL values
> in grass?
> Thanks
> Agus
> Dr. Agustin Lobo
> Instituto de Ciencias de la Tierra (CSIC)
> Lluis Sole Sabaris s/n
> 08028 Barcelona SPAIN
> tel 34 93409 5410
> fax 34 93411 0012
> alobo at ija.csic.es
> http://pangea.ija.csic.es/alobo

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