1. Definition 

Remember parallax? When using a telescoped instrument, parallax occurs when the sighted object and crosshairs don't come to exact foucs on the back of the observer's eye, Figure E-1. Moving the head slightly will cause one image to move across the other.

(a) Initial sight

(b) Eye slightly shifted sideways

Figure E-1 Parallax

We don't want parallax when using a telescope because it causes sighting error BUT in stereophotography, parallax helps us determine point distances. Parallax is the amount a point moves between two photos. 

Figure E-2 shows two points at different elevations appearing on two successive photos.

Figure E-2 Successive Photos

The parallax for each point can be compared by overlaying the photos, Figure E-3.

Figure E-3 Parallax Distances

From Figures E-2 and E-3 it is apparent that parallax is:

• a measurable quantity
• related to elevation: the higher the point, the larger its parallax.

Since parallax can be measured, elevations can be determined provided sufficient support information is available.