Question

In a vertical photograph, which point on the ground lies directly below the camera?

• A. Principal point
• B. Isocenter
• C. Nadir point
• D. Perspective centre

Hint:

Important photogrammetric points: \[ \boxed{ \begin{aligned} \text{Perspective Centre} &\rightarrow \text{Camera lens} \text{Principal Point} &\rightarrow \text{On photograph} \text{Nadir Point} &\rightarrow \text{Ground below camera} \text{Isocenter} &\rightarrow \text{Midpoint between principal and nadir} \end{aligned} } \]

Answer 1

The Correct Option is C

Concept: In aerial photogrammetry, the nadir point is the point on the ground vertically beneath the camera at the instant of exposure.

Step 1: Understand the important points.
• Perspective centre $\rightarrow$ Camera lens centre.
• Principal point $\rightarrow$ Intersection of the optical axis with the photograph.
• Isocenter $\rightarrow$ Point midway between the principal point and the nadir point.
• Nadir point $\rightarrow$ Ground point directly below the camera.

Step 2: Identify the correct point. Therefore, \[ \boxed{\text{The point directly below the camera is the Nadir Point}.} \] Hence, the correct option is \[ \boxed{(C)\;\text{Nadir point}.} \]