In aerial triangulation, the equations used for computing the relative direction generally fall into two categories 1 the complicated mountain equations with unrestricted point-position distribution and 2 the plain equations defined by a 6-fixed-point distribution. These plain equations are not suitable for use in mountainous areas, but this difficulty can be overcome by introducing two additional sets of equations to be used in conjunction with the plain equations. The use of this complex of equations saves computational time and computer memory space compared to the mathematical procedures otherwise required. The additional equations will or will not be used in the computation depending on the magnitude of the point-position error at an image pair. Author
Edited trans. of Ts'e Liang Yu Ti Ch'iu Wu Li Chi K'an (Chinese People's Republic) n1 p57-67 1965.