Question

A camera of 0.20 m focal length was used to take the vertical photograph of a terrain, whose average elevation is 1000 m. In order to get the scale of 1:5000, at what height above the sea level must the aircraft must fly?

• A. 2000 m
• B. 1000 m
• C. 200 m
• D. 5000 m

Hint:

For a vertical photograph the scale is \(\text{Scale} = \dfrac{f}{H - h}\), where \(f\) is the focal length, \(H\) is the flying height above MSL and \(h\) is the average terrain elevation.

Answer 1

The Correct Option is A

Concept:
For a vertical photograph the scale is \(\text{Scale} = \dfrac{f}{H - h}\), where \(f\) is the focal length, \(H\) is the flying height above MSL and \(h\) is the average terrain elevation. We rearrange this to solve for \(H\).

Step 1:
Write the scale as a fraction: \(\dfrac{1}{5000} = \dfrac{0.20}{H - 1000}\). Cross multiplying gives \(H - 1000 = 0.20 \times 5000 = 1000\ \text{m}\).

Step 2:
Add back the terrain elevation to get the height above sea level: \[H = 1000 + 1000 = 2000\ \text{m}.\]

Answer: Option (1) — 2000 m.