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I am newbie in QGIS, but I was wondering how to decrease elevation dem DEM using QGIS? Can QGIS do that? Or anyone of you can tell me any open source GIS software to perform that?

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I'd look at the source data for the DEM. Was it a bunch of points and breaklines you interpolated? How much do you want to decrease the elevation by? Is it a constant value across the surface or only in certain areas? If it's the whole surface, and you have the points in a .CSV file or similar, just open it in Excel, subtract from the elevation field whatever the value is you want to decrease it by, then interpolate your surface again. Another approach is to just do a map algebra operation if it's a raster, subtracting from the grid values whatever the value is you want to reduce the elevation by. I only know about it in ArcMap, but here you can find out about it for QGIS. If it's only a part of the surface you want to decrease the elevation, use QGIS to make a polygon and give the polygon the attribute of the new elevation you want. Then edit the DEM using the polygon as the footprint over which you want to change the elevation. The tool you use to do this should be able to retrieve from the footprint the elevation attribute you gave it. This will of course force a plane at that elevation. But hey, that's a start and you can't expect much more detail given your excessively brief, hastily worded question.

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