|
|
@@ -4,7 +4,7 @@ The function of <em>r.null</em> 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.
|
|
|
+not work with reclassified or external maps.
|
|
|
|
|
|
<p>
|
|
|
The design is flexible. Ranges of values can be set to NULL and/or the NULL
|
|
|
@@ -45,6 +45,15 @@ 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>.
|
|
|
|
|
|
+<h3>External maps</h3>
|
|
|
+From the <a href="r.external.html">r.external</a> documentation: GDAL-linked
|
|
|
+(<em>r.external</em>) maps do not have or use a NULL bitmap, hence <em>r.null</em> 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 <em>r.mask</em>) and
|
|
|
+then "apply" it using e.g. <em>r.resample</em>, or use <em>r.mapcalc</em> to create a copy
|
|
|
+with the appropriate categories changed to NULL (<tt>if()</tt> condition).
|
|
|
+
|
|
|
<h2>EXAMPLES</h2>
|
|
|
Set specific values of a classified map to NULL:<br>
|
|
|
<div class="code"><pre>
|
Ken Mankoff