[Qgis-user] volume calculations in qgis

[Saber Razmjooei]

I had this problem in the past. Unfortunately there are not many plugins
for raster analysis in QGIS and I usually I end up doing in in GRASS.
The command in GRASS for inspection your raster layers, using polygon
vectors:
http://grass.osgeo.org/gdp/html_grass64/v.what.rast.html

You need to run r.null to set the unwanted values to null (i.e. positive
or negative values).


Cheers
Saber

On Tue, 2011-11-15 at 18:57 +0100, Bernhard Draeyer wrote:
> I have 2 high resolution raster DEM's before and after a recent flood 
> event. The aim would be to get for different areas a) erosion/negative 
> volumes b) accumulation/positive volumes and net volume.
> 
> First of all, i subtracted the actual surface from the 'before'-surface 
> to get a difference surface with positive (accumulation) and negative 
> (erosion) values.
> Then i created a shape layer with polygons of different areas of 
> interest. For each of them i need seperate volumetrics. Ideally, the 
> polygons should contain in the end attribute values for accumulation, 
> erosion and net volumes.
> Now i was looking without success for a well adapted method to solve my 
> problem in QGIS or SAGA.
> 
> How should i proceed? Is there any method in QGIS to sum up positive and 
> negative raster values under polygons and put them in seperate attribute 
> fields per polygon? Do i have to convert the raster in a point layer? I 
> saw, that in esri's 3d analyst exist a tin polygon volume function, that 
> covers this task. Does anything similar exist with an os gis?
> 
> 
> Many thanks for your help.
> 
> Bernie
> 
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