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<H2>DESCRIPTION</H2>
The function of <EM>r.null</EM> 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.
<P>
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.
<p>
The <b>setnull</b> 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.
<P>
The <b>null</b> 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.
<H2>NOTES</H2>
Note that value is restricted to integer if the map is an integer map.
<p>
r.null and reclassified maps:<br>
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).
<br>
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:<br>
<tt>r.mapcalc newmap = reclass</tt><br>
Then user recodes this copy.
<H2>SEE ALSO</H2>
<EM><A HREF="r.support.html">r.support</A></EM>
and
<EM><A HREF="r.quant.html">r.quant</A></EM>
<H2>AUTHOR</H2>
U.S.Army Construction Engineering Research Laboratory
<p><i>Last changed: $Date: 2003/05/06 14:12:50 $</i>
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