Relative orientation reproduces the same perspective conditions between a pair of images so that the corresponding light rays in these two photographs are intercepted in space and a stereo model is formed. Following the relative orientation, the process of absolute orientation involves using control points with known horizontal and/or vertical positions to make the stereo model conform in scale and position with respect to the reference plane of the map sheet. At the completion of absolute orientation, the position of any point in the stereo model can be measured at the intersection of two corresponding light rays. .