If you are going to fel a tree, you can predetermine the height, so you know how far the top reaches when he falls. And what he can destroy on his way down.

treeheightInsert a stick into the ground. If the shadow of the stick is as long as the stick, then so is the shadow of the tree as long as the height of the tree. You measure the shade and you know how tall the tree is.
Are shadow and stick not as equally long, they do have the same relationship as the tree and its shadow. With the rule of three, you can then calculate the height.
Length tree / shade tree = your height / own shadow, so
tree length = tree shade X own height / own shadow

Another correct method is based on trigonometry. In a triangle with a right angle and an angle of 45°, the third angle is also 45°, and both short sides are of equal length. We assume that tree and ground form a right angle. Step away from the trunk backwards until you see the top of the tree at 45°. Without tools you can easily estimate the angle by making a kind of a Hitler salute. The height of the tree is now equal to your distance from the tree (plus your eye level, about 160 cm from ground to eye).
You can improve the angle measurement by making a triangle with a right angle with two equal sides in advance, and gluing them together with a look tube as a crosshair on a short and a long side. Trough this you spot the treetop (and your eye level on the trunk on the short under side).
(Of course, there are also professional inclination and altitude measuring devices.)

zwarteCorsicaansedenPossibly you need to adjust for slopes in the area.

You can also approach it as follows: take a stick, vertically, with a horizontally stretched arm. Walk away from the tree until the length of the tree corresponds with that of the stick. Turn the stick horizontally and measure the same length from the base of the tree in the landscape.

Measuring is knowing. A little measure can save you a lot of trouble.