I'd like to have an option to set elevation gain to zero on an activity manually. 

Currently, Strava uses either the device's data or map data. But if both is wrong, there's no way to get meaningful data on the activity.

Typical example - run on a track. Flat as a pancake, but (especially if it's a longer run), it produces some surprising elevation gain numbers. Perhaps because of the arms swing... or because of a hilly area (despite the track is really absolutely flat).

A simple zeroing button/checkbox could be a great solution.