According to Wikipedia, "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 measure distance using trigonometry (stadiametric rangefinders and parallax, or coincidence rangefinders). These methodologies use a set of known information, usually distances or target sizes, to make the measurement, and have been in regular use since the 18th century".
It could be surprising, but probably the Roman Army had a rangefinder. This was the Roman Dodecahedron.
Just recently, I learned about this “mistery” of archaeology: the roman dodecahedron. After preparing a copy of a specific object, I proposed a paper on arXiv, explaining that it can be used for measuring distance (as a telemeter/rangefinder). http://arxiv.org/abs/1204.6497 (In Italiano a http://porto.polito.it/2497004/ )
For me, those dodecahedrons having a structure with holes of different sizes, are military instruments to evaluate distances for ballistics. It is simple to use. Of course, later, during the Middle Age, different instruments had been developed for surveying: the dodecahedron was of the Roman Army, and, probably, its use lost after the collpase of the Empire.