[STATSGRASS] Problems understanding how to use Rpy ( a Python module) to access GRASS rasters in R

[Trevor Wiens]

On Thu, 17 Feb 2005 08:24:05 -0700
William Hudspeth <bhudspeth at edac.unm.edu> wrote:

> Hello,
> 
> I have previously used BASH shell scripts to successfully pass GRASS
> raster data sets to R, and then run basic statistics and plotting
> functions. Now, I am using Rpy 0.4.0 in an application that loads the
> GRASS GIS library. Prior to calling Rpy functions, I make sure to use
> Python to set up all of my environmental variables. I am then able to
> use code such as you see below to load my GRASS location metadata into
> Python objects:
> 
> r_results=r.library('GRASS')
> G=r.gmeta()
> maps=r.get_mapsets()
> raster_layers=r.list_grass(type="cell")
> 
> I understand that the variables on the left are Python objects - I can
> print out correct locational values to screen. However, if I want to
> summarize and plot the values in a certain raster map layer, I know that
> I probably need to create an R object first. It is not clear to me how I
> go about doing this given what I have read in the Rpy documentation. I
> have tried to move a raster into a Python object as such:
> 
> elev=r.rast_get(G,"elevation")
> 
> but, this fails with the error
> 
> sh: line 1: rm command not found
> 

I've done some work with Rpy with varied success. What I have often found difficult is the translation betwen Python objects and R objects with can be controlled with

    mode = get_default_mode()
    set_default_mode(NO_CONVERSION)

so you can set it back later. This is important for sum functions. I've not tried to use the GRASS library through Rpy. 

There is also an Rpy mailing list which I read and their may be individuals on that list who may be able to help. Roger had suggested simple examples, and I have found those types of tasks, regression, AIC model selection and such work fine along with many plotting tasks in conjunction with pyPgSQL.

T
-- 
Trevor Wiens 
twiens at interbaun.com

The significant problems that we face cannot be solved at the same 
level of thinking we were at when we created them. 
(Albert Einstein)