There is an application, in which we don’t eliminate the distortion effect of the relief but, on the contrary, we use it for our purposes. The so-called anaglif image can be constructed for a section area of the aerial photographs, taken from different positions. The black-and-white versions of the two images are turned to different colors and a unified color image is compiled from them. If this image is observed through eyeglasses with the same colors used for the anaglif, it appears as a three-dimensional image in our brains (Fig. 62).
There is nothing more to do than processing both images. However, at the ortho-rectification step, we shall use the same horizontal planes as an elevation model for both images. It should be repeated: at this step only; for the estimation of the external element parameters, the vertical positions of the control points should be known. While displaying the anaglif, it is important that the different colors in the print and the eyeglass should be in same order (e.g. the red on the right, the green on the left both in the images and at the eyeglass). Otherwise, the image shown three-dimensional character only after a rotation by 180 degrees.