The condition that a point in object space, the corresponding point in image space, and the corresponding perspective centre must be collinear (i.e., all lie on the same straight line). This is similar to the co planarity condition, which states a relationship between three lines and one plane. The two types of condition are theoretically equivalent, but it has been found that in practise collinearity conditions are easier to use for solving photogrammetric problems than are co planarity conditions.