comparison of data

Malcolm Williamson malcolm at
Mon Mar 22 09:55:35 EST 1993

Cathy Ciret writes:
> Hi
> I,m a new user of GRASS,and I'd like to be able to compare maps (with
> the same categorie values).Does anybody know how to do it?
> I've got map of vegetation with ,for example 5 or 10 classes,and
> I'd like to get the DIFFERENCE between those maps.It 's look
> easy to combine or overlay map in Grass,but I just can't find
> useful commands to make comparison...
> Thanks .
Welcome to the world of GRASS! First of all, almost all analyses in GRASS are 
performed in the raster domain, so if your data is not in raster format it must
become so ( Be sure that your resolution (defined by g.region) is 
acceptable. The single most powerful tool in GRASS is r.mapcalc - take the time
to grow familiar with it. To find the difference between two map layers, you
will use a syntax something like the following:

> newmap = map1 - map2

newmap will be a separate map layer.

You will find it a worthy investment of your time to read the brief description
of each GRASS command in the GRASS manual, either in hardcopy or online 
(g.manual). I would recommend looling over all of the "r.______" commands (the
raster commands).

If you have any specific questions, feel free to e-mail me directly.

Malcolm D. Williamson - Technical Assistant      E-mail: malcolm at
Center for Advanced Spatial Technologies      Telephone: (501) 575-5736
Ozark Rm. 214                                       Fax: (501) 575-3846 
University of Arkansas              
Fayetteville, AR 72701

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