Rangefinder

A rangefinder is a device that measures distance from the observer to a target.

Some devices use active methods to measure (such as ultrasonic ranging module, laser rangefinder, radar distance measurement); 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.