Aster DEMs are in EPSG 4326 (lat lon WGS84). According to the Gdal DEM documentation

"For LatLong projections near the equator, where units of latitude and units of longitude are similar, elevation (z) units can be converted to be compatible by using scale=370400 (if elevation is in feet) or scale=111120 (if elevation is in meters). For locations not near the equator, it would be best to reproject your grid using gdalwarp before using gdaldem."

In this answer for a problem with hillshading (Scale and Z factor have no effect on hillshade analysis in QGIS) user Luke provides a way to calculate the correct "ratio of vertical to horizontal units" factor by multiplying the cosine of the latitude times 111320. This factor also affects slope calculations.

In the image I was able to produce slopes in degrees from a DEM in EPSG4326 at latitudes ranging from 19°45N and 20°N. If you want to work with a DEM in a CRS that defines the measuring units of the coordinates and the elevations in meters you must reproject the dem using raster/projections/warp (reproject)

In the image I want to reproject from EPSG 4326 (lat lon wgs84) to EPSG 32164 (UTM WGS84 14N). When calculating the slope the "ratio of vertical to horizontal units" must be set to 1.

Hope it helps

Aster DEMs are in EPSG 4326 (lat lon WGS84). According to the GDAL DEM documentation:

For LatLong projections near the equator, where units of latitude and units of longitude are similar, elevation (z) units can be converted to be compatible by using scale = 370400 (if elevation is in feet) or scale = 111120 (if elevation is in meters). For locations not near the equator, it would be best to reproject your grid using gdalwarp before using gdaldem.

In this answer for a problem with hillshading (Scale and Z factor have no effect on hillshade analysis in QGIS) user @Luke provides a way to calculate the correct scale ("ratio of vertical to horizontal units") by multiplying the cosine of the latitude times 111320. The scale affects slope calculations.

In the image below I was able to produce slopes in degrees from a DEM in EPSG 4326 at latitudes ranging from 19°45N and 20°N. The scale (-s) was set to 104550.67.

If you want to work with a DEM in a CRS that defines the measuring units of the coordinates and the elevations in meters you must reproject the DEM using raster/projections/warp (reproject). For example, reprojecting from EPSG 4326 (lat lon wgs84) to EPSG 32164 (UTM WGS84 14N). See below:

In this case, when calculating slope, the "ratio of vertical to horizontal units" (scale) would be set to 1.

Aster DEMs are in EPSG 4326 (lat lon WGS84). According to the Gdal DEM documentation

"For LatLong projections near the equator, where units of latitude and units of longitude are similar, elevation (z) units can be converted to be compatible by using scale=370400 (if elevation is in feet) or scale=111120 (if elevation is in meters). For locations not near the equator, it would be best to reproject your grid using gdalwarp before using gdaldem."

In this answer for a problem with hillshading (Scale and Z factor have no effect on hillshade analysis in QGIS) user Luke provides a way to calculate the correct "ratio of vertical to horizontal units" factor by multiplying the cosine of the latitude times 111120. This factor also affects slope calculations.

In the image I was able to produce slopes in degrees from a DEM in EPSG4326 at latitudes ranging from 19°45N and 20°N. If you want to work with a DEM in a CRS that defines the measuring units of the coordinates and the elevations in meters you must reproject the dem using raster/projections/warp (reproject)

In the image I want to reproject from EPSG 4326 (lat lon wgs84) to EPSG 32164 (UTM WGS84 14N). When calculating the slope the "ratio of vertical to horizontal units" must be set to 1.

Hope it helps

Aster DEMs are in EPSG 4326 (lat lon WGS84). According to the Gdal DEM documentation

"For LatLong projections near the equator, where units of latitude and units of longitude are similar, elevation (z) units can be converted to be compatible by using scale=370400 (if elevation is in feet) or scale=111120 (if elevation is in meters). For locations not near the equator, it would be best to reproject your grid using gdalwarp before using gdaldem."

In this answer for a problem with hillshading (Scale and Z factor have no effect on hillshade analysis in QGIS) user Luke provides a way to calculate the correct "ratio of vertical to horizontal units" factor by multiplying the cosine of the latitude times 111320. This factor also affects slope calculations.

In the image I was able to produce slopes in degrees from a DEM in EPSG4326 at latitudes ranging from 19°45N and 20°N. If you want to work with a DEM in a CRS that defines the measuring units of the coordinates and the elevations in meters you must reproject the dem using raster/projections/warp (reproject)

In the image I want to reproject from EPSG 4326 (lat lon wgs84) to EPSG 32164 (UTM WGS84 14N). When calculating the slope the "ratio of vertical to horizontal units" must be set to 1.

Hope it helps