Figure 11.

Horizon-ratio scale from motion parallax. In this diagram, the line e represents an extent in the environment. The point of observation moves parallel to e, from H to H’, through an extent v. Successive lines of regard to a point P on the horizon are parallel to each other, thus establishing at every distance a scale factor equal to v; thus, the extent f is equal to v. If the visual angles E and F are reasonably small, then e/v ≈E/F. (From Sedgwick, 1973.)