The legal altitude limit in France is 120m, measured as the shortest path between the drone and the terrain. I can set the max. altitude in the DJI Fly app as 500m, which is still within the rules at half-height of an 1300m high mountain (as I'm only 15-20m above the trees). As shown on the fig. above, the RTH without altitude change would come back above me in a level path, which would violate laws very quickly.