Rangefinder

Originally optical devices used in surveying, they soon found applications in other fields, such as photography, the military, and space travel.

The word telemeter is derived from Ancient Greek τῆλε (têle) 'distant, far away' and μέτρον (métron) 'something used to measure'.

At the point to be measured, two sliding targets on a surveyor's rod were adjusted to align with the hairs in the telescope.

The Royal Society of Arts gave an award to W. Green for its invention in 1778, even though they were made aware of Watt's priority.

Two mirrors set a distance apart horizontally in a long slim box, similar to a subtense bar but located at the measuring station, and forming two images.

It used the measurement of the angle of depression from the observer, sited on a high vantage point, to the waterline of the target vessel.

[7] World War II-era rangefinders worked optically with two telescopes focused on the same target but a distance apart along a baseline.