What are internal guiding elements and external guiding elements?

The internal orientation element is a parameter that determines the relationship between the camera lens center (strictly speaking, it should be the image node of the lens) and the image position. It includes three parameters: the positions x and y of the main point of the image (the vertical foot of the main optical axis on the image plane) relative to the image center, and the vertical distance f (the main distance) from the lens center to the image plane, which is generally determined by camera calibration.

The external orientation element is a parameter that determines the spatial position and posture of the image or the main beam of photography at the moment of photography, including six parameters. Three of them describe the position of the photography center in the object coordinate system X, Y and Z; Three parameters Ω, Φ and κ that describe the attitude.