Question

A square grid is laid on a flat terrain and is photographed from an aerial camera. The flying height and camera parameters are assumed to be constant. The camera and lens are assumed to be perfect (i.e. free from any distortions). The image of the grid obtained from the camera is shown below. Select the CORRECT statement from the statements given below. 

• A. Camera is looking directly downwards (towards nadir)
• B. The given photograph is a vertical photograph
• C. The given photograph is an oblique photograph
• D. Scale over the given photograph is constant

Hint:

Parallel lines staying parallel and nearly uniform scale $⇒$ vertical photo (flat terrain). Converging lines/foreshortening $⇒$ oblique photo.

Answer 1

The Correct Option is C

In a vertical photograph of a flat terrain (optical axis vertical and image plane parallel to ground) parallel grid lines remain parallel and, neglecting lens distortion, the scale is uniform. The shown image exhibits perspective convergence/foreshortening (grid lines meeting toward a vanishing point), which results from camera tilt. Therefore it is an oblique photograph. Hence (C) is correct, while (A) and (B) are false. Statement (D) is false because scale is not constant in an oblique photo.