DESCRIPTION
The function of r.null is to explicitly create the NULL-value
bitmap file. The intended usage is to update maps that do not have a
NULL-value bitmap file (i.e. to indicate for each pixel if zero is a valid
value or is to be considered as NULL, i.e. no data value). The module does
not work with reclassified maps.
The design is flexible. Ranges of values can be set to NULL and/or the NULL
value can be eliminated and replace with a specified value.
The setnull parameter is used to specify values in the ranges to
be set to NULL. A range is either a single value (e.g., 5.3), or a pair of
values (e.g., 4.76-34.56). Existing NULL-values are left NULL, unless the
null argument is requested.
The null parameter eliminates the NULL value and replaces it with
value. This argument is applied only to existing NULL values, and not to the
NULLs created by the setnull argument.
NOTES
Note that the value is restricted to integer if the map is an integer map.
r.null and reclassified maps
r.null does not support reclassified maps because, if r.null
was run on the reclass raster it would alter the original and any other
reclass rasters of the original. Therefore r.null does not allow
recoding reclassified maps (products of r.reclass).
As a workaround, the way to recode such a map is: The user creates a raster
map out of the reclassified map by copying it:
r.mapcalc "newmap = reclass"
NULL data compression
By default no data files (i.e., NULL files) are not compressed unless a
specific environment variable is set. The NULL file compression must be
explicitly turned on with export GRASS_COMPRESS_NULLS=1.
Warning: such raster maps can then only be opened with GRASS GIS 7.2.0 or
later. NULL file compression can be managed with r.null -z.
EXAMPLES
Set specific values of a classified map to NULL:
r.null map=landcover.30m setnull=21,22
Set NULL-values of a map to a specific value:
r.null map=fields null=99
SEE ALSO
r.compress,
r.support,
r.quant
AUTHOR
U.S.Army Construction Engineering Research Laboratory
Last changed: $Date$