2. 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 F-2. Moving the head slightly will cause one image to move across the other.
(a) Initial sight |
(b) Eye slightly shifted sideways |
Figure F-2 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 F-3 shows two points at different elevations appearing on two successive photos.
Figure F-3 Successive Photos |
The parallax for each point can be compared by overlaying the photos, Figure F-4.
Figure F-4 Parallax Distances |
From Figures F-3 and F-4 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.