What is the main point of an image and which letter is commonly used in photogrammetry stereograms?

1. It is a science and technology to study and determine the shape, size, position, nature and relationship of the photographed object by using the photos taken by an optical camera.

2. Let the aerial camera lens be point S, the plane where photos are taken is p, and the shooting place is approximate plane E. This vertical line is called the vertical line, the intersection point A between the vertical line and P is called the image bottom point, and S is the vertical line of P. When P intersects with O, O is called the image main point.