Question

For a proper stereoscopic viewing, the stereopair should be oriented in such a way that the flight line of each photo lies ___________.

• A. along a common straight line
• B. on two mutually perpendicular planes
• C. on two parallel straight lines with a horizontal gap equal to half of the size of the photo
• D. on two mutually perpendicular lines

Hint:

Think about which alignment of the two photos removes cross-track (y) parallax so the eyes can fuse the stereo model.

Answer 1

The Correct Option is A

Step 1: Recall the goal of relative orientation for stereoscopic viewing.
When two overlapping aerial photographs are placed under a stereoscope for three dimensional viewing, they must first be relatively oriented, that is, positioned with respect to each other, so that corresponding rays from the two photographs intersect properly and the observer's eyes can fuse the two images into a single stereo model without strain.
Step 2: Recall what a flight line represents on each photo.
The flight line (or air base line) on each photograph is the line joining that photo's own principal point with the conjugate principal point transferred from the adjacent photo, that is, the trace of the direction of flight on that photograph. It represents the direction along which the aircraft was travelling when both exposures were taken.
Step 3: State the orientation condition.
For proper stereoscopic viewing, the two photographs of the pair must be laid out on the table (or in the mount) such that the flight line marked on the left photo and the flight line marked on the right photo both lie along one and the same common straight line. This removes the y-parallax (any perpendicular, cross-track displacement) between corresponding image points, since a genuine perspective pair of photos taken from a straight flight path should have their flight lines collinear when correctly oriented. Only after this condition is satisfied does the floating mark move smoothly, and only in the flight direction, as the parallax bar is adjusted.
Step 4: Eliminate the other options.
Two mutually perpendicular planes (option B) or perpendicular lines (option D) would introduce, not remove, cross parallax and make stereo fusion impossible; these do not describe correct orientation. A fixed horizontal gap equal to half the photo size (option C) is not a required orientation rule at all, the actual base distance between principal points on the table depends on the model scale chosen, not a fixed half-photo-size rule, and this description does not by itself guarantee the flight lines are aligned.
\[ \boxed{\text{Option (A): along a common straight line}} \]