From OpenStreetMap Wiki
|The accumulated ascent for any kind of non-physical route.|
|Used on these elements|
|Status: in use|
|Tools for this tag|
- ascent=ascent in m is used to indicate the total accumulated ascent in meters of a route relation.
Ascent should not be used on single ways, because it will cause confusion if the way is split up into several parts.
If you want to use another unit=km/m/miles/mph/... use ascent=ascent value plus unit=km/m/miles/mph/... separating the unit from the ascent key. This makes it easier for handling the data.This is not accepted tagging practice and is not widely used. Instead, units are commonly given as part of a value, like maxspeed=30 mph or height=2 m.