Without a frame of reference it is very hard to estimate the height of something and if you are planing on cutting down a tree it is a good thing to know how far it will fall. The process I’m going to describe to you is based on mathematics which gives it a surprising amount of accuracy. Don’t worry you do not need to know the trigonometry behind it to make it work, there are just a few key points to making it work.
First of all this is based on a right or square angle, if the tree or the stick you hold are not vertical it will throw off your estimate. But as long as the tree doesn’t have a significant lean you are good.
Second this is most accurate when you look at a point at the base of the tree that is the same distance off the ground as your eye is. You can then just add how high your eye is to the height you estimate. However as long as you are not looking at a small tree sighting at the ground is only a small error.
Third if you are on sloping ground that will also throw off your estimate.
All you need is a stick as long as the distance from your eye to your outstretched hand. Then hold the stick vertical with your arm outstretched and try to have the base of the stick level with your eye. This means your arm will be angled up slightly. Next walk closer or further from the tree until the stick appears to be the same height as the tree. Once the stick and the tree match to your eye you are as far away from the tree as it is tall. If you just want to know how far out it will go when you cut it down you are done, the top of the tree should land where you are standing. If you need to know numbers, measure from where you are standing to the tree and that gives you the height.