[STATSGRASS] R & GRASS inicialization.

[Roger Bivand]

On Fri, 2 Aug 2002, andres javier garcia garcia wrote:

> Hello all;
> I've just installed the R interface to GRASS. (I'm new to R), and I'm not 
> sure if it is properly installed.
> From GRASS y type R, and
> >system("g.list rast") shows my raster maps properly.
> Then I've tried to follow the steps indicated in Bivand, (2000). And typed
> >tapply(dem, cover,summary)
> (where dem is a Digital Elevation Model, and cover is a map of different 
> clases of vegetal cover)
> and the following message appears:
>  Object "cover" not found.
>   

The interface expects you first to collect the grass metadata (current 
region):

G <- gmeta()

You can then do

summary(G)

to check that it is OK.

I'm now assuming that you have a number of raster layers in GRASS, among 
them dem and cover. I think you did:

dem <- rast.get(G, rlist=c("dem", "cover"), catlabels=c(FALSE, TRUE))

and tried to work on dem. If you do

str(dem)

you will see that it is a list with two members, dem and cover. To get at 
the actual data, you need say:

myrasts <- rast.get(G, rlist=c("dem", "cover"), catlabels=c(FALSE, TRUE))

then

tapply(myrasts$dem, myrasts$cover,summary)

NB: for tapply() to work, cover needs to be a "factor" in R terminology. 
To get some insight into the meaning of lists, factors, etc in R, Agus' 
advice is the best way to go.

> I've tried the same with different location and mapsets and none works.

Please let us know how you get on!

Roger

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand at nhh.no
and: Department of Geography and Regional Development, University of
Gdansk, al. Mar. J. Pilsudskiego 46, PL-81 378 Gdynia, Poland.