Alternatively, you can create a very accurate measuring device with a spirit level and two short pieces of wood. With the spirit level at the base form a triangle with the other two pieces of wood so that the main viewing angle is 45 degrees.
Holding the spirit level straight at your eye level, move backwards from the base of the tree, looking up the viewing side until the top of the tree appears in your line of sight. The trees height is the distance you are from its base plus the height off the ground of the spirit level.