altitude hold fail

Does anyone know what the altitude limit is when using "terrain follow" mode? Lets say I wanted it to stay 100 ft of the ground and there is a fairly steep hill with 70 ft trees on it (not too thick), would the trees interfere with the sensors? Or is it more GPS? Thanks for your replies.

I have experienced this 2x times now using tripod mode. First time I was flying in the snow with obstacle avoidance system on. Mavic would have a hard time staying in a straight line when moving forward and it would go slightly up and down on altitude randomly (minor variance) Second time was...