r.null manual: better explain NULL compression; manual cleanup

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@70218 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.null/r.null.html

@@ -1,9 +1,10 @@
 <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.
+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.
 
 <p>
 The design is flexible. Ranges of values can be set to NULL and/or the NULL
@@ -18,6 +19,32 @@ 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 the value is restricted to integer if the map is an integer map.
+
+<h3>r.null and reclassified maps</h3>
+
+<em>r.null</em> does not support reclassified maps because, if <em>r.null</em>
+was run on the reclass raster it would alter the original and any other
+reclass rasters of the original.  Therefore <em>r.null</em> does not allow
+recoding reclassified maps (products of <em>r.reclass</em>).
+<br>
+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:<br>
+
+<div class="code"><pre>
+r.mapcalc "newmap = reclass"
+</pre></div>
+
+<h3>NULL data compression</h3>
+
+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 <tt>export GRASS_COMPRESS_NULLS=1</tt>.<br>
+Warning: such raster maps can then only be opened with GRASS GIS 7.2.0 or
+later. NULL file compression can be managed with <b>r.null -z</b>.
+
 <h2>EXAMPLES</h2>
 Set specific values of a classified map to NULL:<br>
 <div class="code"><pre>
@@ -30,27 +57,13 @@ r.null map=fields null=99
 </pre></div>
 
 
-<h2>NOTES</h2>
-
-Note that value is restricted to integer if the map is an integer map. 
-<p><em>r.null</em> 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 recoding 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>
-
-<div class="code"><pre>
-r.mapcalc newmap = reclass
-</pre></div>
-
-
 <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>
+<em>
+<a href="r.compress.html">r.compress</a>,
+<a href="r.support.html">r.support</a>,
+<a href="r.quant.html">r.quant</a>
+</em>
 
 <h2>AUTHOR</h2>