The total descent of a route is the sum of all negative elevation differences — in other words, all the meters you descend along the route. For example: imagine you start at an altitude of 1000 meters, then descend two mountains of 1000 meters each, and end again on a mountain top at 1000 meters. Your total descent would then be 2000 meters, even though you finish at the same elevation where you started.
Supportcenter