A rangefiner 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.