How to Find a Tree's Height
The Shadow Method
This article outlines a very basic method to measure the height of a tree (within about 10% accuracy). This could definitely be a fun activity for the kids while hiking through a forest. If you think you've found a tree over 378 feet, than you may have found the world's largest living tree!
The first thing to do is to measure your own shadow. If you have a tape measure, then great. Otherwise, measure how many steps it takes to walk the shadow. Of course, you probably shouldn't move while measuring so either ask a friend to do the measuring or keep your eyes fixed on the end of the shadow.
The next thing is to measure the shadow of the tree. If it's not too long a shadow, you can walk heel-to-toe in the same way that you measured your own shadow. However, if it's a long shadow, you can walk at a moderate/slow pace (not heel-to-toe) and multiply the number of steps you take by 2 to estimate the number of heel-to-toe steps you would have taken.
Finally divide the tree shadow step number by your own shadow step number to find the ratio of the heights. Multiply this ratio by your own height to find the tree's height.
Example: Your shadow was 10 (heel-to-toe) steps long. The tree's shadow was 80 steps (at a slow pace). Your height is 5 ft.
Solution: The tree's shadow is 80*2 = 160 heel-to-toe steps. The ratio of your heights is 160/10 = 16. Multiply this by your height to get 16* 5 = 80 ft, the height of the tree.