The MASK is only applied when reading an existing GRASS raster map, for example when used in a module as an input map. The MASK will block out certain areas of a raster map from analysis and/or display, by "hiding" them from sight of other GRASS modules. Data falling within the bounaries of the MASK can be modified and operated upon by other GRASS raster modules; data falling outside the MASK is treated as if it were NULL.
Because the MASK is actually only a reclass map named "MASK", it can be copied, renamed, removed, and used in analyses, just like other GRASS raster map layers. The user should be aware that a MASK remains in place until a user renames it to something other than "MASK", or removes it using "r.mask -r" or g.remove.
Grid cells in the MASK map containing 0 or NULL will replace data with NULL, while cells containing other values will allow data to pass through unaltered.
To restore raster operations to normal (i.e., all cells of the current region), remove the MASK file by setting the -r remove MASK flag. In this case, a dummy value must also be given for the input parameter. A MASK also can be removed by using g.remove or by renaming it to any other name with g.rename.
The MASK is read as an integer map. If MASK is actually a floating-point map, the values will be converted to integers using the map's quantisation rules (this defaults to round-to-nearest, but can be changed with r.quant).
r.mask uses r.reclass to create a reclassification of an existing raster map and name it MASK. A reclass map takes up less space, but is affected by any changes to the underlying map from which it was created. The user can select category values from the input raster to use in the MASK with the maskcats parameter; if r.mask is run from the command line, the category values listed in maskcats must be quoted (see example below).
Somewhat similar program functions to those performed by r.mask can be done using r.mapcalc, g.region, and other commands.
Create a raster mask, for contraining the calculation of univariate statistics of the elevation values for "lakes":
# set computation region to lakes raster map g.region raster=lakes -p # use lakes as MASK r.mask raster=lakes # get statistics for elevation pixels of lakes: r.univar elevation
g.region raster=geology_30m -p r.category geology_30m d.mon wx0 d.rast geology_30m r.mask raster=geology_30m maskcats="217 thru 720" d.mon wx0 d.rast geology_30m
Last changed: $Date: 2014-12-19 13:17:36 -0800 (Fri, 19 Dec 2014) $
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