A Rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure (such as sonar, laser, or radar); others, available since the nineteenth century, measure distance using trigonometry (stadiametric rangefinders and parallax, or coincidence rangefinders). They usually use a set of known distances or target sizes to make the measurement.
Rangefinders may be used by military or law enforcement snipers as a means of calculating the distance to the target to set up a perfect shot. The laser rangefinder is not always the best option though, as it sends out a light source that may give away the snipers' position.