The condition placed, in photogrammetry, on three lines the line joining the perspective centres at the times of exposure of two photographs, and the two lines joining the point in object space to the corresponding points in image space that these lines be co planar. Also spelled co-planar. Denoting the lines by vectors y, x₁, and x₂, condition is x₁ x x₂ , y = 0. Although this condition has been used by several organ-izations as the basis for computer programs, it is not as versatile nor as widely used as the co linearity condition.