[GRASS-user] Re: r.mapcalc changes color table

[Glynn Clements]

Hermann Peifer wrote:

> >> I have a large georeferenced digital map.
> >>
> >> I successfully imported it into GRASS.
> >>
> >> Now I would like to "cut" only a small rectangle -- my investigation
> >> area -- out of this large map.
> >>
> >> I used the following approach:
> >>
> >> 1) import the TIFF
> >> 2) select the part of interest in the GRASS monitor
> >> 3) set the zoomed area as region (monitor tools)
> >> 4) create a boundary vector: v.in.region
> >> 5) convert boundary raster to a raster v.to.rast
> >> 6) use r.mapcalc to cut: r.mapcalc "cut=if(boundary_raster, large_map,
> >> null())"
> > 
> > Replace step 6 with:
> > 
> > 	g.copy rast=boundary_raster,MASK
> > 	r.resample input=large_map output=cut
> > 	g.remove rast=MASK
> > 
> 
> Can someone perhaps explain why steps 4 and 5 are necessary?
> (My understanding is that r.mapcalc anyway observes the region settings)

I wasn't paying attention. For a rectangular area, you can just change
the region then make a copy with r.resample or r.mapcalc. I was
assuming an arbitrary (not necessarily rectangular) region.

For a rectangular region, steps 3 through 6 can be replaced with a
single call to r.resample.

> Couldn't step 6 be simplified to:  r.mapcalc cut = large_map
> ?

Which is equivalent to "r.resample in=large_map out=cut". Using
r.resample is probably more convenient from the GUI, as you can select
the input from a list.

-- 
Glynn Clements <glynn at gclements.plus.com>