[GRASS-user] r.update

[Dave Roberts]

Friends,

     After some alternative approaches to a simple problem of 
compositing rasters (see previous posts, esp. compositing rasters @9:32)
I developed a very crude function to update rasters

r.update target mask x y

updates raster target by substituting y everywhere that raster mask has 
x.  A more GRASS-like syntax would be

r.update target=string mask=string current=integer replace=integer

but my bash skills are pretty limited and I elected (for the time being) 
not to parse the arguments that way.  The current function relies on a 
short bash script and a FORTRAN executable.  The crude part is exporting 
both the target an mask rasters as ascii exports, creating a new ascii 
file, and doing an r.in.ascii to bring the updated raster back in.  It 
only works (at present) for integer rasters, but it's intended for 
thematic maps, so that seems OK.

r.update.sh just below.

r.out.ascii inp=$1 out=$1.asc null=-1
r.out.ascii inp=$2 out=$2.asc null=-1
g.remove rast=$1
r_update $1.asc $2.asc $3 $4
r.in.ascii  inp=tmp.file out=$1 nv=-1
rm $1.asc
rm $2.asc
rm tmp.file

You have to "alias r_update 'sh r_update.sh'" or name the script 
r.update and chmod +x r.update to make it run as shown.

The FORTRAN code for r_update is at

ecology.msu.montana.edu/GRASS/r_update.f90

The code compiles with gfortran.  The executable must be in your path, 
or you can modify the bash script with a full path to it.

     It is my sincere hope that someone with GRASS chops will write a 
function for g.extension that uses the API and avoids all this ascii 
input and output, but this serves as a demo and useful (if clumsy) 
approach in the meantime.

Dave
-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
David W. Roberts                                     office 406-994-4548
Professor and Head                                      FAX 406-994-3190
Department of Ecology                         email droberts at montana.edu
Montana State University
Bozeman, MT 59717-3460