I'm passing an SRTM1 3601x3601 file to "gdaldem slope".....I've queried the values in Python, they are reasonable (min 222, max 424) for an area of SW Iowa. Took a bit to get the syntax right, but file is being created all right. Oddly though, almost all cells have a value from say 89.95 to 89.99, with a few having zeros (and the borders, no surprise / no problem, 9999). Looking at the 0's, it is when the elevation is flat, say for a water body.
Default slope is supposed to be degrees; anyone have any idea why I am getting the ~89.9x values?