Scouts measure height for First Class rank:
First Class Requirement 2
Using a map and compass, complete an orienteering course that covers at least one mile and requires measuring the height and/or width of designated items (tree, tower, canyon, ditch, etc.).
Peter Le Roux at the Ropes and Poles Blog provides the helpful illustration above and the directions:
This technique for measuring the height of an object is an old one- there is a very similar drawing in Scouting For Boys. In principle, it is very simple: by measuring the distance on the ground, we can calculate the height of the tree by comparing it to the height measured on a stave.
In this simplified metric version:
Measure of 9 units (they can be anything- meters, 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