Question

In aerial photogrammetry, the scale of a vertical photograph is given by:

• A. Flying height / focal length
• B. Focal length / flying height
• C. Flying height $\times$ focal length
• D. Flying height – focal length

Hint:

Scale is never perfectly uniform across an aerial photograph if the terrain is not perfectly flat.
The scale at a specific point depends on its elevation; higher points on the ground appear at a larger scale than lower points.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Scale in photogrammetry is the ratio of a distance on the photograph (map distance) to the corresponding distance on the ground (ground distance).
Step 2: Key Formula or Approach:
For a truly vertical photograph taken over flat terrain, the scale (\( S \)) is defined as:
\[ S = \frac{\text{Distance on Photo (d)}}{\text{Distance on Ground (D)}} = \frac{f}{H - h} \]
Where:
\( f \) = Focal length of the camera lens.
\( H \) = Flying height of the aircraft above the Mean Sea Level (MSL).
\( h \) = Elevation of the ground above the MSL.
Step 3: Detailed Explanation:
If we consider the flying height relative to the ground (let \( H_{avg} = H - h \)), the formula simplifies to:
\[ S = \frac{f}{H_{avg}} \]
This shows that the scale is directly proportional to the focal length and inversely proportional to the flying height.
A shorter focal length or a higher flying altitude results in a smaller scale (covers more area but with less detail).
Step 4: Final Answer:
The scale of a vertical photograph is the ratio of focal length to flying height.