floating mark

A mark seen as occupying a position in the three-dimensional space formed by the stereoscopic fusion of a photograph and used as a reference mark in examining or measuring the stereoscopic model. The marks may be formed by one real mark lying in the projected object space or by two real marks lying in the projected or virtually projected spaces of two photographs. These marks also may be formed by two real marks lying in the planes of the photographs themselves or by two virtual marks lying in the image planes of the binocular-viewing apparatus.

The principle of the floating mark. If the mark in the left of diagram is adjusted on the point m3, m1 and m2, the observer perceives it as if the floating mark were moved to the points M3, M1 and M2. The mark on the right is assumed to be constant. The floating mark chiefly moves vertically but also laterally.