Put the plane mirror on the ground, with the tree on one side of the plane mirror and the person on the other side.
The person steps back until he can just see the top of the tree in the plane mirror.
Then measure the distance M1 from the person to the plane mirror, and the distance M2 from the tree to the plane mirror.
Then measure the vertical distance H1 from the human eye to the ground.
(Cannot be calculated using a person’s height)
Expression H=M2*H1/M1