[GRASS-user] calculate the volume of a DEM

Mon Jul 9 21:04:52 PDT 2018

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<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www at 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 at wsfrf1420" type="cite"> 
I am trying to calculate the volume of a DEM using GRASS. 
 
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>
 
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.

<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www at wsfrf1420" type="cite"> 
For that I first calculated the area of my DEM using s.surf.area . 
The result was 349 492.7m². 
Then I used r.univar to get the number of cells, it is 822 244. 
So the cell size is 349 492/822 244 = 0,42m² 
 
You can get the cell size simply from r.info 
Then r.univar gives the total of the cell values in it's "sum" output. 
 
So putting it all together, i.e.: 
 
<tt>GRASS 7.4.0 (ITM):~ > r.info -g faran_lidar | grep "res"</tt> 
<tt>nsres=1</tt> 
<tt>ewres=1</tt> 
<tt># Cell size of 1 meter</tt> 
 
<tt>GRASS 7.4.0 (ITM):~ > r.info -r faran_lidar</tt> 
<tt>min=73.83</tt> 
<tt>max=297.64</tt> 
<tt># Min max values</tt></blockquote>
 
I don't get what this last command is for? 
 
 
Suppose I want the volume above 150 meters: 
 
<tt># Always remember to set region</tt> 
<tt>GRASS 7.4.0 (ITM):~ > g.region -p rast=faran_lidar</tt> 
<tt># Prepare the "cut" raster between 150 and the DEM</tt> 
<tt>r.mapcalc "faran_cut = if(faran_lidar>150, faran_lidar, null())"</tt> 
<tt># And get the sum of all cells:</tt> 
<tt>GRASS 7.4.0 (ITM):~ > r.univar faran_cut | grep sum</tt> 
<tt> 100%</tt> 
<tt>sum: 840009490.273132</tt> 
<tt># Total 840 million cubic meters</tt> 
 
Done 
 
 
 
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) 
 
I just have to multiply the "sum" result with the cell size? 
Because you wrote that it is in cubic meters, but I though it was just in meters

<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www at wsfrf1420" type="cite"> 
To get the volume, I want to multiply this cell size with the total elevation value of my DEM. Where can I find this value? Is it the "sum" at the end of the r.univar report?</blockquote>
Yes

<blockquote cite="mid:790575905.2053.1531105524458.JavaMail.www at wsfrf1420" type="cite"><br>
<span style="font-size:12px">Otherwise, do you think</span><span style="font-size:11px"><span style="font-size:12px"> my</span></span><span style="font-size:12px"> reasonning is correct to calculate the volume of a DEM?<br>
<br>
Thank you for your help,<br>
Marion</span><br>
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Thank you for your help

<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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