The New Student's Reference Work/Telemeter

Telem′eter, an instrument for finding the distance of an inaccessible object. In military circles this instrument is generally called a range-finder. Nearly all telemeters are based upon one of two general principles: (1) The image of a distant object, produced by a lens, is geometrically similar to the object itself and depends for its size upon the distance of the object; (2) having chosen a convenient base-line of known length, it is possible by measuring the angle between this base and the direction of the distant object (first at one end of the baseline and then at the other) to compute the distance of the object. A great variety of these instruments have been invented, some adapted to infantry in the field and therefore compact and less accurate, others adapted to naval operations in which much larger base-lines and hence much greater accuracy can be obtained.

A similar process when employed by surveyors is called stadia-measurement.