gdal - How do I know the vertical/horizontal units of my DEM? I'm creating a slope using gdaldem

I'm using gdaldem to create a slope map.

I can apply a scale. The default is 1.

The notes say:

Ratio of vertical units to horizontal.
If horizontal unit of source DEM is degrees (e.g WGS84 projection), use scale=111120
if the vertical units are meters (or scale=370400 if they are in feet)

My DEM is in NAD27 UTM 18N and in the projection properties it says linear units Meters and angular units degrees.

How do I know what the vertical units are?

Answer

Your horizontal units are metres (= linear units). You can not derive your z-value units (vertical units) from the coordinate system. It's like checking the coordinate system properties to try and work out if a temperature raster z-values are in centigrade, kelvin or fahrenheit. You need to go back to the metadata, if there is any.

You can probably assume your DEM z-values are metres, but I've been caught with that before when I didn't check the range of the values/metadata and they turned out to be in centimetres...