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<p>Retested on two small regions, one with some null cells and one
with no null cells, using both the GRASS r.covar module and the R
layerStats function, and this time results were almost identical.
In addition, r.covar outputs "N" the number of cells used, and I
saw that in the case of null cells indeed the value of N was
smaller. So that answers my previous post :-)</p>
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<p>Now I redo the process on the full rasters to see what caused the
different in results.</p>
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<p>Regards, Micha</p>
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<div class="moz-cite-prefix">On 11/19/18 12:41 PM, Markus Neteler
wrote:<br>
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<blockquote type="cite"
cite="mid:CALFmHhsKV2YKdEzh8efsRfDvhOHoWMFtFFou3WczEs5D93gEVg@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">On Sun, Nov 18, 2018 at 10:32 PM Micha Silver <a class="moz-txt-link-rfc2396E" href="mailto:tsvibar@gmail.com"><tsvibar@gmail.com></a> wrote:
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<pre class="moz-quote-pre" wrap="">
I am preparing a correlation matrix for 7 raster layers. The results using the r.covar module are different from the R layerStats function. I suspect this is due to handling of null cells. The R function has a parameter to remove NA cells, but the GRASS module, I think, just loops over all cells, including no value.
Can anyone confirm that GRASS does not deal with null cells, and that this would cause the difference in correlation results?
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<pre class="moz-quote-pre" wrap="">
Here how r.covar treats NULL cells:
<a class="moz-txt-link-freetext" href="https://trac.osgeo.org/grass/browser/grass/trunk/raster/r.covar/main.c#L93">https://trac.osgeo.org/grass/browser/grass/trunk/raster/r.covar/main.c#L93</a>
Markus
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<pre class="moz-signature" cols="72">--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918</pre>
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