A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, navigation, determining focus in photography, choosing a golf club, or accurately aiming a weapon. Some devices use active methods to measure (such as sonar, laser, or radar); others measure distance using trigonometry (stadiametric rangefinders and parallax, or coincidence rangefinders). Older methodologies that use a set of known information, usually distances or target sizes, to make the measurement, have been in regular use since the 18th century. Wikipedia, Rangefinding
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