In this simplified metric version:
Measure of 9 units (they can be anything- metres, feet, stave lengths, even Scout lengths)
Place a stave upright on that point, and measure one more unit.
Placing your head on the ground, look up to the top of the tree. Make a mark on the stave. The height of the tree will be ten times the height marked on the stave.
This can be fairly accurate, providing that the tree is roughly upright and the ground is basically level
drawn on iPad Mini using Paper app and Just-Mobile AluPen