[Qgis-user] volume calculations in qgis

[Bernhard Draeyer]

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