I'm attempting a simple slope analysis in QGIS2 using the plugin at Raster -> Terrain Analysis -> Slope. The slope output layer has a very strange histogram, basically spikes at 0 and at 89.9098 with one big valley in the middle:
Here's the initial DEM:
And the resulting slope layer (note that Contrast Enhancement is already at 'stretch to minmax'):
The place I got the DEM is the NED. I tried the gridfloat format, and the arcgrid format, and the IMG format. I downloaded from nationalmap and also from atlas.ca.gov - all seem to have the same resulting histogram.
The procedure I used to make the slope layer was Raster -> Terrain Analysis -> Slope, GeoTIFF, Z factor = 1.0 (what does that do anyway?)
Seems like simple pilot error; however: I found this tutorial online (for QGIS 1.something) that comes with a sample DEM, and it worked just fine using the same procedure. So, it's not a fundamental QGIS2 bug.
So, it must be slightly more complex pilot error: there must be a conversion step or something I'm missing?