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# Photogrammetry - Stereoscopic Parallax

Parallax
Apparent shift in the position of an object, with respect to a frame of reference, caused by a shift in the position of observation.
Change in position of an image from one photo to the next is caused by aircraft’s motion.
– Called stereoscopic parallax, x parallax, or simply parallax
Parallax of any point is directly related to the elevation of the point.
Parallax is greater for high points than for low points.

Stereoscopic Parallax
Stereoscopic parallax is caused by taking photographs of the same object but from different points of observation.
Note the displacement between the top and base of the storage towers in this photo stereo-pair.

Parallax of Principal Point
Parallax of left ground principal point is photo base b’ measured on right photo, and the parallax of right ground principal point is photo base b measured on left photo.
For moderate relief, b ≈ b’
Photo base is average of two values

Photographic Flight-Line Axes for Parallax Measurement
Parallax is occurred along the flight-line.
Photographic x and x′ axes for parallax measurement must be parallel with the flight line.
Flight line is the line connecting the two principal points.
The y and y′ axes for parallax measurement pass through their respective principal points and are perpendicular to the flight line.

Monoscopic Methods of Parallax Measurement
Direct measurement of x and x' on the left and right photos, respectively.
By fastening the photographs down on a table:
flight lines O_1 O_2 and O_1'O_2' are marked as usual.
Now parallax is P_b=D-d_(b )

Stereoscopic Methods of Parallax Measurement
Stereoscope is used with a parallax bar, or a stereometer for calculation.
D = spacing between principal points
C= the parallax bar constant.
K = the distance from the fixed mark to the index mark of the parallax.
Parallax pf Point A =P_a=X_a'-X_a=D-(K-r_a)=(D-K)+r_a

Parallax
Apparent shift in the position of an object, with respect to a frame of reference, caused by a shift in the position of observation.
Change in position of an image from one photo to the next is caused by aircraft’s motion.
– Called stereoscopic parallax, x parallax, or simply parallax
Parallax of any point is directly related to the elevation of the point.
Parallax is greater for high points than for low points.

Stereoscopic Parallax
Stereoscopic parallax is caused by taking photographs of the same object but from different points of observation.
Note the displacement between the top and base of the storage towers in this photo stereo-pair.

Parallax of Principal Point
Parallax of left ground principal point is photo base b’ measured on right photo, and the parallax of right ground principal point is photo base b measured on left photo.
For moderate relief, b ≈ b’
Photo base is average of two values

Photographic Flight-Line Axes for Parallax Measurement
Parallax is occurred along the flight-line.
Photographic x and x′ axes for parallax measurement must be parallel with the flight line.
Flight line is the line connecting the two principal points.
The y and y′ axes for parallax measurement pass through their respective principal points and are perpendicular to the flight line.

Monoscopic Methods of Parallax Measurement
Direct measurement of x and x' on the left and right photos, respectively.
By fastening the photographs down on a table:
flight lines O_1 O_2 and O_1'O_2' are marked as usual.
Now parallax is P_b=D-d_(b )

Stereoscopic Methods of Parallax Measurement
Stereoscope is used with a parallax bar, or a stereometer for calculation.
D = spacing between principal points
C= the parallax bar constant.
K = the distance from the fixed mark to the index mark of the parallax.
Parallax pf Point A =P_a=X_a'-X_a=D-(K-r_a)=(D-K)+r_a