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2. Parallax 
The separation between corresponding points in similar images when super 
imposed . 
3. Registration 
The act of transforming one or both of a pair of similar images so as to 
reduce all parallaxes to zero when the images are superimposed. 
4. Relative Distortion 
A difference in size or shape of similar images such that a transformation 
of one or both images is required to achieve registration. 
Relative image distortions are familiar in photogrammetric practice. Con 
vergent and panoramic photography probably represent the most extreme cases of 
such distortions, with convergent-panoramic stereograms being particularly 
interesting. 
It can be shown that plane projective distortion contains no orders higher 
than the second except for the effects of earth curvature and atmospheric re 
fraction. Projective distortion as encountered in the keystoning of convergent 
stereo photographs contains uniform second-order distortion components over the 
entire overlap area. Toward the edges of the field, however, first-order scale 
and skew distortions become predominant. 
Panoramic photographs taken in their entirety present considerable third 
and higher order distortions. Sections of panoramic photographs of a size 
likely to be examined at one time, however, show much less higher order dis 
tortion, and consequently it is not proposed to consider transformations of 
orders higher than the second when dealing with such photography. 
Terrain relief also introduces higher order relative distortion between 
stereo pairs. Such distortions and displacements are, of course, in the X 
direction only and are not subject to systematic transformation by either 
optical or electronic means. 
5. Manual Registration 
Manual registration refers to the visual observation of parallax and the 
manual adjustment of the various image transformations as required to reduce 
parallaxes to zero. 
Manual registration operations are common in photogrammetry as, for exam 
ple, in the relative orientation of stereo pairs. Relative orientation in 
volves first- and second-order transformations with five degrees of freedom. 
When one considers the time and tedium involved in the process of relative 
orientation, it is appreciated that a manual registration in which all of the 
12 prime transformations are separately subject to control would be, for most 
purposes, impractical. Also, the complexity of a mechanical-optical instru 
ment providing a separate adjustment for each of the 12 transformations in 
volved would, we believe, be equally impractical.