GRASS Raster Mask

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I don't understand the concept of the MASK. What is it?


A MASK allows you to block arbitrary areas of all raster maps processed while the MASK is set. That is, those cells for which the MASK is set, will be treated as if they were NULL. Originally, it was important when GRASS didn't have a true concept for the NULL value (GRASS 4.x). It is, however, still very useful in all GRASS versions (5.x, 6.x and 7.x). MASKing can be even used inside a script. This way, a raster MASK can be applied to vector features that need to be masked. This is achievable by using the masked y|n switch.