I'm trying to calculate the slope for a raster layer using ArcMap 10.2 and the Slope (Spatial Analyst) function. The raster I'm using is an NED 10m mosaic DEM, and the mosaic's datum is in WGS 1984. I am trying to calculate the degree slope, but my numbers are wonky. I know the output shapefile for the slope is incorrect because, according to these calculations, known flat areas have a slope of 71.9 - 89.9 degrees. My Z factor is 1 currently. I've tried reprojecting my DEM to NAD 1983 (2011) and change the Z factor, but I got the same results. Does anyone know what the problem I'm encountering is, and if so, how do I fix it?
One thing to note: I noticed my mosaic does not have an established Linear Unit (Layer Properties > Source > Spatial Reference (scroll)), which I found odd. Not sure if that ties into my problem or not, but I figured I'd mention it.