Rangefinders are used to measure the distance between the observed object and the observer, they are used everywhere where it is important to know the exact values of distances, whether in surveying, construction, for accurate focusing of lenses during photography or accurate adjustment of the shot of weapons or sports such as golf.
Rangefinders differ in their measuring technologies and can be basically divided into passive and active. Active rangefinders use some kind of transmitted energy, laser, sound (sonar), etc. to measure distances. Passive ones are nowadays more of a curiosity and a mathematical and geometric "game" where one uses easily measurable angles and a single distance on the ground to calculate the necessary lengths, called trigonometry.