Article Index

 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


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.


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