Discussion
What will be the focal length of the camera when the scale of the aerial photograph is 1:4500, the flying height of the aircraft is 3780 meters above MSL and the average height of the terrain is 820 meters above sea level ?
Answer: B
The focal length is 657.8 mm.
The scale of a vertical aerial photograph is the focal length divided by the flying height above the ground:
\[ \begin{aligned} S &= \frac{f}{H - h} \\ H - h &= 3780 - 820 = 2960\ \text{m} \\ f &= \frac{2960}{4500}\ \text{m} = 0.6578\ \text{m} \end{aligned} \]
So f = 657.8 mm. The common error is to use the flying height above sea level: \(3780/4500 = 0.84\) m, which is not offered. The other options do not come from this formula with the given heights.
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