Back to GRASS: simple mapcalc issue

A. Giacomelli andreag at
Wed Apr 5 03:46:58 EDT 2000

Dear list,

after a long time "off GRASS" (and using other nastier packages) I have
a chance to work again with this GREAT PIECE OF SOFTWARE. My
congratulations to all the people who are maintaining GRASS.

...I have a problem which should be a trivial issue for more recently
trained users:

I would simply like to calculate the sum of two rasters r1 and r2
(suppose these are rasters with two areas -which partially overlay- and
"null" values around). In my grass4.1 memories, The result  of "r1 +r2"
should return 

1- the sum of r1 and r2 where the rasters overlay, 
2- the values of r1 (or r2) where they do not overlay.

r.mapcalc in grass 5 does (1) but returns Null instead of (2)...

I was thinking of overcoming this behaviour with some logical operators
or other things, but there must be a much simpler way...
(I remember some documentation explaining how Null behaves, but I can't
find it now on the web site).

Regards and TIA for any feedback,

Andrea Giacomelli
Centre for Advanced Studies, Research and
Development in Sardinia
Environment Group

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