[GRASS-user] Cropping maps

[Moritz Lennert]

On 26/09/17 16:17, Moritz Lennert wrote:
> On 26/09/17 15:57, Marco Alicera wrote:
>> Dear list,
>> I am trying to crop a raster map into a smaller area, but I am failing.
>>
>> This wiki page link <https://grasswiki.osgeo.org/wiki/Cropping_maps> is
>> quite clear:
>> "Make boundary into mask using r.mask
>> <https://grass.osgeo.org/grass72/manuals/r.mask.html> then copy raster
>> source to new map (g.copy
>> <https://grass.osgeo.org/grass72/manuals/g.copy.html>). All cells
>> outside of the mask will be omitted from the new copy."
>>
>> r.mask vector=myarea
>> g.copy --overwrite raster=OldRasterWithBigArea,NewRasterWithSmallArea
>> r.mask -r
>>
>> I expected all cell outside were not going to be omitted, but replaced
>> by NULL.
> 
> I'm not sure I understand your last sentence, but AFAIK, the wiki info
> is wrong. g.copy works directly on the files and ignores mask and region
> settings. In your example, OldRasterWithBigArea and
> NewRasterWithSmallArea should be strictly identical.
> 
> You need to use r.mapcalc "new = old" while the mask is set to get a
> raster with everything outside the mask set to NULL.

I've rewritten the wiki page to try to make things clearer.

Moritz