What do you mean by external directional elements?

Externally oriented elements are also called externally oriented elements. The basic data to determine the geometric relationship of the object photographic beam. It is used to characterize the spatial position of the photographic beam at the moment of photography, including the three-dimensional coordinate values (x, y, z) of the photographic center in a certain spatial rectangular coordinate system and six data of three angular orientation elements that determine the spatial orientation of the photographic beam. Among them, the three angular orientation elements are often represented by different elements because of the different rotation angle systems: when the rotation angle system with Z axis as the main axis is adopted, the azimuth T of the main vertical plane, the tilt angle of the photo and the rotation angle C of the photo are represented; When the rotation angle system with Y axis as the main axis is adopted, it is expressed as heading inclination ψ, lateral inclination W and photo rotation angle X; When the rotation angle system with X axis as the main axis is adopted, it is expressed as lateral inclination W', heading inclination ψ' and photo rotation angle K'.