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   The function of r.null is to explicitly create the NULL-value bitmap
   file. The intended usage is to fix "old" maps that don't have a
   NULL-value bitmap file (i.e. to indicate if zero is valid value or is
   to be converted to NULL). 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.

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   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

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   Note that value is restricted to integer if the map is an integer map.

   r.null and reclassified maps:
   The problem is, 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 doesn't allow to recode reclassified maps (products of
   r.reclass).
   So, the way to recode such a map is: The user makes a raster out of the
   reclass that isn't a reclass by copying it:

r.mapcalc newmap = reclass

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   [1]r.support and [2]r.quant

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   U.S.Army Construction Engineering Research Laboratory

   Last changed: $Date: 2006-01-06 03:31:35 -0600 (ÐÑ, 06 Ñнв 2006) $

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References

   1. file://localhost/root/tmp/2/fin/r.support.html
   2. file://localhost/root/tmp/2/fin/r.quant.html