I got that hillshade from my DHM (I used the hillshade-module from qgis). For my purpose there a too much relief details (roughness). How can I get rid of them in QGIS or GRASS?
Which hillshade module, Martin? In QGIS Master there are two under the Raster menu:
Raster -> Terrain analysis -> Hillshade
Raster -> Analysis -> DEM(Terrain models)
If you haven't already done so, I should try the second one (set 'Mode' to 'Hillshade'). Also, tick the 'Creation Options' box and enter 'TFW' under 'Name' and 'YES' under 'Value'. Doing this will make a world file for the hillshade tiff, which means that you will be able to edit it in GIMP or whatever to do things like applying blur, without losing the georeferencing (provided that you don't change the image size, that is). Nick.
If your DHM is a ESRI ASCII grid file, you should definitly give Dr. Jennys Terrain Sculptor a try: http://terraincartography.com/terrainsculptor/!
Look also at this post for converting to ESRI ascii grid: http://freegeographytools.com/2010/converting-dem-files-to-asc-format-for-terrain-bender-creating-matching-raster-overlays