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: Recall what scale means in photogrammetry. 
In photogrammetry, the scale of a photograph expresses how a measured length on the photo compares with the actual length on the ground.
It is defined as the ratio of photo distance to the corresponding ground distance.

Step 2: Write the scale relationship for a vertical photograph.
For a truly vertical aerial photograph taken over level terrain, the scale $S$ is given by:

\[ S = \frac{\text{photo distance }(d)}{\text{ground distance }(D)} \]

This ratio can also be expressed in terms of camera and flight parameters as:

\[ S = \frac{f}{H - h} \]

where:

• $f$ is the focal length of the camera,
• $H$ is the flying height above mean sea level,
• $h$ is the elevation of the ground above mean sea level.

Step 3: Use flying height above ground.
If the aircraft height is measured directly above the ground surface, let:

\[ H_{\text{ground}} = H - h \]

Then the scale relation simplifies to:

\[ S = \frac{f}{H_{\text{ground}}} \]

This shows that the scale increases with focal length and decreases as the flying height increases.
Higher altitude or shorter focal length results in a smaller-scale photograph.

Step 4: Final conclusion.
The scale of a vertical aerial photograph is equal to the ratio of the camera focal length to the flying height above the ground.