How to measure distance with fingers and eyes

Theoretically, straighten your arm and give your thumb a thumbs up, and make a similar calculation according to the distance from your eyes to your thumb (about the length of your arm), the length of your thumb and the height of the measured object. But in fact, when using this skill, it is basically based on experience, and it takes a long time of practice to master it skillfully. What needs to be corrected is that finger ranging is mostly used for marching and artillery positioning rough measurement, which is a compulsory course for every soldier. Snipers rarely measure the distance with their fingers, because it is easy to expose themselves by straightening their arms, and snipers often use visual inspection directly. Finger ranging can generally estimate 2-4 kilometers (when there are obvious objects, such as houses and trees), and the error of experienced soldiers is less than 200 meters. Visual inspection is generally used to estimate the distance within one kilometer, with an error of 50 meters.