r.mask(1)

NAME
r.mask - Creates a raster mask for limiting raster operation.

KEYWORDS
raster, mask, null data, no-data

SYNOPSIS
r.mask
r.mask --help
r.mask [-ir] [raster=name] [maskcats=string] [vector=name] [layer=string] [cats=range] [where=sql_query] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

  Flags:

-i
    Create inverse mask

-r
    Remove existing mask (overrides other options)

--overwrite
    Allow output files to overwrite existing files

--help
    Print usage summary

--verbose
    Verbose module output

--quiet
    Quiet module output

--ui
    Force launching GUI dialog

  Parameters:

raster=name
    Name of raster map to use as mask

maskcats=string
    Raster values to use for mask
    Format: 1 2 3 thru 7 *
    Default: *

vector=name
    Name of vector map to use as mask
    Or data source for direct OGR access

layer=string
    Layer number or name (vector)
    Vector features can have category values in different layers. This number determines which layer to use. When used with direct OGR access this is the layer name.
    Default: 1

cats=range
    Category values (vector)
    Example: 1,3,7-9,13

where=sql_query
    WHERE conditions of SQL statement without ’where’ keyword (vector)
    Example: income < 1000 and population >= 10000

DESCRIPTION
r.mask facilitates the creation and management of a raster mask to control raster operations. While the computational region specifies the extent (rectangular bounding box) and resolution, the mask specifies the area that should be considered for the operations and area which should be ignored. The mask is represented as a raster map called MASK by default.

The mask is applied when reading an existing GRASS raster map, for example when used as an input map in a module. 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 boundaries 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.

By default, r.mask converts any non-NULL value in the input map, including zero, to 1. All these areas will be part of the mask (see the notes for more details). To only convert specific values (or range of values) to 1 and the rest to NULL, use the maskcats parameter.

Because the mask raster map created with r.mask is actually only a reclass map named MASK by default, it can be copied, renamed, removed, and used in analyses, just like other GRASS raster maps.

The user should be aware that a mask remains in place until it is removed or renamed. To remove a mask and restore raster operations to normal (i.e., all cells of the current region), remove the mask by setting the -r flag (r.mask -r). Alternatively, a mask can be removed using g.remove or by renaming it to any other name with g.rename.

The GRASS_MASK environment variable can be used to specify the raster map which will be used as a mask. If the environment variable is not defined, the name MASK is used instead.

NOTES
The above method for specifying a "mask" may seem counterintuitive. Areas inside the mask are not hidden; areas outside the mask will be ignored until the mask is removed.

r.mask uses r.reclass to create a reclassification of an existing raster map and names it MASK by default. 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). Note that the maskcats can only be used if the input map is an integer map.

  Different ways to create a mask
Mask can be created using r.mask or by creating a mask raster map directly.

The r.mask tool creates a mask with values 1 and NULL. By default, r.mask converts all non-NULL cells to 1. If a raster with ones (1) and NULLs values is used with r.mask, all raster cells with value 1 will be included in the computation, while those with NULL will be masked out.

r.mapcalc -s "map1 = if(row() < col(), 1, null())"
r.mask raster=map1

If a binary map with zeros and ones as values is used with r.mask, and both NULLs and zeros should be masked out, the maskcats parameter can be used to specify the values that should be masked out:

r.mapcalc -s "map2 = if(row() < col(), 1, 0)"
r.mask raster=map2 maskcats="0"

Mask can also be created directly using any tools that have a raster as output, by naming the output raster MASK (or whatever the GRASS_MASK environment variable is set to). Both NULLs and zeros will be masked out in the raster mask. This allows for creation of a simple binary raster with only ones and zeros where cells with zeros in the mask raster are excluded from the computation (behaving as if they were NULL). A raster with zeros and ones can be created and used directly as a mask by naming the output raster MASK, e.g., using raster algebra:

r.mapcalc -s "MASK = if(row() < col(), 1, 0)"

  Handling of floating-point maps
r.mask treats floating-point maps the same as integer maps (except that floating maps are not allowed in combination with the maskcats parameter); all non-NULL values of the input raster map are converted to 1 and are thus part of the mask. In the example below, all raster cells are part of the mask, i.e., nothing is blocked out from analysis and/or display.

r.mapcalc "map3 = rand(0.0,1.0)"
r.mask raster=map3

However, when using another method than r.mask to create a mask, the user should be aware that the mask is read as an integer map. If mask is 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.mapcalc "map4 = rand(0.0,1.0)"
g.copy raster=map4,MASK

In the example above, raster cells with a rounded value of 1 are part of the mask, while raster cells with a rounded value of 0 are converted to NULL and consequently blocked out from analysis and/or display.

EXAMPLES
The examples are based on the North Carolina sample dataset.

Create a raster mask, for constraining 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

Remove the raster mask with the -r flag:

r.mask -r

Creating a mask from selected categories in the North Carolina ’geology_30m’ raster map:

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

SEE ALSO
g.region, r.mapcalc, r.reclass, g.remove, g.rename, r.quant

AUTHOR
Michael Barton, Arizona State University

SOURCE CODE

Available at: r.mask source code (history)

Accessed: Monday May 11 17:55:44 2026

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