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Document r.external issues with r.null #91

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Sep 1, 2019
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Document r.external issues with r.null #91

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11 changes: 10 additions & 1 deletion raster/r.null/r.null.html
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Expand Up @@ -4,7 +4,7 @@ <h2>DESCRIPTION</h2>
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
Expand Down Expand Up @@ -45,6 +45,15 @@ <h3>NULL data compression</h3>
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
(r.external) maps do not have or use a NULL bitmap, hence r.null cannot
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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
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then "apply" it using e.g. r.resample, or use r.mapcalc to create a copy
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with the appropriate categories changed to NULL (if() condition).
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<h2>EXAMPLES</h2>
Set specific values of a classified map to NULL:<br>
<div class="code"><pre>
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