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 > > _______________________________________________ > Qgis-user mailing list > Qgis-user@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/qgis-user _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user