Question

Consider a pair of overlapping vertical aerial photographs taken from a flying height of 665 m above a point A on the ground, with a camera having a focal length of 152.4 mm. The height of the point A above the mean sea level is 535 m. The parallax bar reading of the point A as measured from the photographs is 10.96 mm. Assuming the air base to be 400 m, the parallax bar constant is _________ mm.

• A. 80.71
• B. 457.96
• C. 39.84
• D. 24.18

Hint:

Parallax bar constant is computed using $\frac{Bf}{H - h}$. Always ensure units are consistent, and convert focal length to meters if the base and heights are in meters.

Answer 1

The Correct Option is A

The formula for the parallax bar constant (\(k\)) is:

\[ k = \frac{B \cdot f}{H - h} \] Where:
- \( B = \text{air base} = 400 \, \text{m} \)
- \( f = \text{focal length} = 152.4 \, \text{mm} = 0.1524 \, \text{m} \)
- \( H = \text{flying height above MSL} = 665 \, \text{m} \)
- \( h = \text{height of point A above MSL} = 535 \, \text{m} \)

\[ k = \frac{400 \times 0.1524}{665 - 535} = \frac{60.96}{130} \approx 0.468 \, \text{mm per mm parallax} \]

Now, using the measured parallax \( p = 10.96 \, \text{mm} \), we compute the parallax bar constant (PBC):

\[ \text{PBC} = p \times \left( \frac{B \cdot f}{H - h} \right) = 10.96 \times 0.468 \approx 80.71 \, \text{mm} \]