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Can I run the GRASS command 'r.patch' on raster files which are not "loaded" into the current GRASS session? That is, rasters which are simply in my folder but not in a GRASS location & mapset? Thanks.

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No, to work with raster files in GRASS you have to import the rasters with or you can link external raster files with r.external:

r.external input=/path/to/raster1.tif output=raster1
r.external input=/path/to/raster2.tif output=raster2
g.region rast=raster1,raster2 -p
r.patch input=raster1,raster2 output=raster_patched
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Edit: I have added the g.region command to set the computational region ( correctly prior to the patching. – markusN Apr 3 '12 at 9:24

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