This question already has an answer here:
I have a DEM that was made from UAV/photogrammetry techniques by Maps Made easy.
In QGIS I loaded the DEM and then made a slope raster layer.
My values go from 0 to a huge number like 23456e+07. Does that seem right?
The highest slope on the terrain is not much more than 28%.
I think the layer is spitting out wrong data. Do I need to enter a Z factor?
Also, when I enter my formula in raster calculator
("slope@1" > 28) * 0 + ("slope@1" < 28) * 1 it turns my whole image black.
My end goal is to display a map with the georeferenced jpeg with areas over 28% slope shaded in red.