[GRASSLIST:2544] Moire effect in aspect map

[Christof Bigler]

I imported a DEM with r.in.gdal and derived aspect with r.slope.aspect 
(floating point). These raster maps have a slightly different 
resolution (27.28 m) than the default settings (30 m).
When I look at the aspect map, it looks ok on the screen and the 
printout as well. However, when I zoom in, I can detect a sort of moire 
effect (diagonal bands crossing the map). Interpolation didn't have any 
effects.
Any ideas how to avoid this?

Thanks in advance.

Christof