Final answer:
In aerial photography, the nadir is the spot directly under the camera, representing the closest surface point perpendicular to the camera's line of sight.
Step-by-step explanation:
In aerial photography, the spot directly under the camera is referred to as the nadir. This point is significant because it represents the part of the Earth's surface that is closest to the camera and is perpendicular to the camera's line of sight straight down. Understanding the concept of nadir is crucial when assessing the perspective and coverage of aerial photographs. The quality and accuracy of images in aerial photography largely depend on the position of the camera relative to the subject at the moment the shutter opens and closes.