I have a DEM as a Geotiff and I have conducted slope analysis in QGIS. I've done this many times and everything was ok but this time I received reversed values: 0 for max slope and 89 for min slope. I have no idea what is wrong. Geotiff was generated from .asc files and seems to be ok.
I suppose there is something wrong with the first Geotiff file. I took "native" .asc file and made slope analysis without a problem. After that I generated Geotiff from the Slope.asc and is also fine.