Question

A parallax bar can be used to measure ____________ while viewing the two photos stereoscopically.

• A. parallax
• B. flying height
• C. scale of the photo
• D. focal length of the camera

Hint:

Recall what a parallax bar physically reads when its floating mark is set on a point viewed stereoscopically.

Answer 1

The Correct Option is A

Step 1: Understand what a parallax bar does.
A parallax bar (also called a stereometer) is a hand instrument used with a stereoscope to measure the small difference in position of corresponding image points on two overlapping aerial photographs. When two photographs are placed in a stereo pair and viewed through a stereoscope, the observer sees a fused three dimensional model of the terrain. The parallax bar has two glass plates, each engraved with a small mark (a dot or a cross). One mark is fixed and the other can be moved along a graduated scale using a micrometer screw.
Step 2: Recall the meaning of parallax in photogrammetry.
When the aircraft moves between exposures, an object point that is closer to the camera (or higher in elevation) shifts by a different amount on the two photographs than an object point that is farther away (or lower in elevation). This apparent displacement of an image point along the direction of flight, between the left photo and the right photo of a stereo pair, is called the x-parallax (or absolute stereoscopic parallax), given by \( p = x_a - x_b' \), where \(x_a\) is measured on the left photo and \(x_b'\) is measured on the right photo, both from their respective principal points along the flight direction.
Step 3: Explain how the parallax bar reads this quantity.
By placing the fixed mark of the parallax bar on the image of a point on one photograph and the movable mark on the same point on the other photograph, and then turning the micrometer screw until the two marks appear to fuse into a single floating mark that touches the ground point in three dimensional stereo view, the reading on the parallax bar scale directly gives the parallax of that point. The difference between the parallax readings of two points is what is used, through the parallax equation \( h = H - \dfrac{fB}{p} \), to compute elevations from a stereo pair.
Step 4: Eliminate the other options.
Flying height (option B) is obtained from camera and mission parameters, not from a parallax bar directly. Photo scale (option C) is the ratio of a photo distance to the corresponding ground distance and depends on focal length and flying height, not on a parallax bar reading. Focal length (option D) is a fixed camera constant supplied by the camera calibration certificate, again not measured with a parallax bar. Only the stereoscopic parallax is the quantity a parallax bar is designed to read.
\[ \boxed{\text{Option (A): parallax}} \]