<span style="font-family:arial,helvetica,sans-serif; font-size:12px">Thank you
for this clarification, one last question:<br>
the cell size is nsres*ewres right?</span>
<div class="gl_quote" style="margin-top: 20px; padding-top: 5px;">De : "Micha
Silver"<br>
A : marion-bru...@sfr.fr<br>
Envoyé: mardi 10 juillet 2018 21:06<br>
Objet : Re: [GRASS-user] calculate the volume of a DEM<br>
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<div class="moz-cite-prefix">On 07/10/2018 06:05 AM, <a
class="moz-txt-link-abbreviated"
href="mailto:marion-bru...@sfr.fr";>marion-bru...@sfr.fr</a> wrote:</div>
<blockquote cite="mid:1392724731.1181.1531191958732.JavaMail.www@wsfrf1420"
type="cite"><br>
<span style="font-family:arial,helvetica,sans-serif; font-size:12px"></span>
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<div class="gl_quoted">
<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www@wsfrf1420"
type="cite"><span style="font-family:arial,helvetica,sans-serif"><span
style="font-size:12px">Hello,</span></span><br>
Hi</blockquote>
Hello again,
<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www@wsfrf1420"
type="cite"><br>
<span style="font-family:arial,helvetica,sans-serif"><span
style="font-size:12px">I am trying to calculate the volume of a DEM using
GRASS.</span></span><br>
<br>
A DEM is usually elevation above sea level, so the "volume of a DEM" would be
the the total volume from sea level to the DEM elevation. Is that what you
want? Usually you want the volume between two rasters: the DEM and some base
elevation. So you need to begin with a simple r.mapcalc expression to get the
difference bwteen the DEM and the base elevation.</blockquote>
<br>
My bad, I chose the wrong words. I had two DEMS and I used r.mapcalc to get the
difference between them. I want to calculate the total volume of this last
raster.</div>
</div>
</blockquote>
<br>
Ah, OK, it's clear now.<br>
<blockquote cite="mid:1392724731.1181.1531191958732.JavaMail.www@wsfrf1420"
type="cite">
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<div class="gl_quoted">
<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www@wsfrf1420"
type="cite"><br>
<span style="font-size:12px"><span
style="font-family:arial,helvetica,sans-serif">For </span></span><span
style="font-family:arial,helvetica,sans-serif"><span
style="font-size:11px"><span style="font-size:12px">that</span></span><span
style="font-size:12px"> I first calculated the area of my DEM using
s.surf.</span><span style="font-size:11px"><span style="font-size:12px">area
.</span></span></span><br>
<span style="font-size:12px"><span
style="font-family:arial,helvetica,sans-serif">The result was </span>349
492.7m².<br>
Then I used r.univar to get the number of cells, it is 822 244.<br>
So the cell size is 349 492/822 244 = 0,42m²</span><br>
<br>
You can get the cell size simply from r.info<br>
Then r.univar gives the total of the cell values in it's "sum" output.<br>
<br>
So putting it all together, i.e.:<br>
<br>
<tt>GRASS 7.4.0 (ITM):~ > r.info -g faran_lidar | grep "res"</tt><br>
<tt>nsres=1</tt><br>
<tt>ewres=1</tt><br>
<tt># Cell size of 1 meter</tt><br>
<br>
<tt>GRASS 7.4.0 (ITM):~ > r.info -r faran_lidar</tt><br>
<tt>min=73.83</tt><br>
<tt>max=297.64</tt><br>
<tt># Min max values</tt></blockquote>
<br>
I don't get what this last command is for?</div>
</div>
</blockquote>
Just to see the range of values<br>
<blockquote cite="mid:1392724731.1181.1531191958732.JavaMail.www@wsfrf1420"
type="cite">
<div class="gl_quote" style="margin-top: 20px; padding-top: 5px;">
<div class="gl_quoted"><br>
<br>
<em>Suppose I want the volume above 150 meters:<br>
<br>
<tt># Always remember to set region</tt><br>
<tt>GRASS 7.4.0 (ITM):~ > g.region -p rast=faran_lidar</tt><br>
<tt># Prepare the "cut" raster between 150 and the DEM</tt><br>
<tt>r.mapcalc "faran_cut = if(faran_lidar>150, faran_lidar, null())"</tt><br>
<tt># And get the sum of all cells:</tt><br>
<tt>GRASS 7.4.0 (ITM):~ > r.univar faran_cut | grep sum</tt><br>
<tt> 100%</tt><br>
<tt>sum: 840009490.273132</tt></em><br>
<tt><em># Total 840 million cubic meters</em></tt><br>
<br>
<em>Done</em><br>
<br>
<br>
<br>
So if do r.univar on my raster that I calculated with r.mapcalc before, it is
enough? Or do I have to redo all the steps above? (I also set the region
before)<br>
<br>
I just have to multiply the "sum" result with the cell size?<br>
Because you wrote that it is in cubic meters, but I though it was just in
meters</div>
</div>
</blockquote>
<br>
r.univar is all you need, but yes you must multiply by the cell size. If your
DEM is meters and your cell size is 10 meters, then the r.univar sum result is
in units of 10x10x1 = 100 cubic meters. So you would multiply the sum by 100 to
get cubic meters.<br>
<blockquote cite="mid:1392724731.1181.1531191958732.JavaMail.www@wsfrf1420"
type="cite">
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<div class="gl_quoted"> </div>
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</blockquote>
<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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