## 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 or external 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.

### External maps

From the r.external documentation: GDAL-linked (r.external) maps do not have or use a NULL bitmap, hence r.null cannot manipulate them directly. Here NULL cells are those whose value matches the value reported by the GDALGetRasterNoDataValue() function. To apply the GDAL-linked the user need to either create a MASK (e.g. with r.mask) and then "apply" it using e.g. r.resample, or use r.mapcalc to create a copy with the appropriate categories changed to NULL (`if()` condition).

## 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