Stereoscopy and Orthoimagery

If you place your thumb in front of you and then look at it with your left eye and then the right eye, what you will notice is that the thumb is “moving” in relation to the objects behind it. This phenomenon is what helps the human brain see depth and thus perceive which object is in front and [Page 2694]which one behind. It is called binocular vision, and the displacement of your thumb is called parallax. The larger the parallax, the further our thumb is from the objects behind it. What we actually perceive is the parallax, and through this we see and appreciate depth. However, it would be wrong to think that all human depth perception comes from the parallax. There are many more cues, such as shading, relative size, precedence, contrast, and haze, that work just fine with monocular vision.

Figure 1 Relative photogrammetric accuracy versus angle of intersection

Source: Light, D. L., Brown, D., Colvocoresses, A., Doyle, F., Davies, M., Ellasal, A., et al. (1980). Satellite photogrammetry. In C. C. Salma, C. Theurer, & S. W. Henriksen (Eds.), Manual of photogrammetry (4th ed., pp. 1305–1369). Falls Church, VA: American Society for Photogrammetry. Reprinted by permission of the American Society of Photogrammetry and Remote Sensing.

This impression of depth can be also realized with the use of images of the same object that are taken from different positions. When each of a pair of such images is separated and viewed by each eye separately, it is possible to perceive depth. This is called stereoscopic vision or stereo-vision. There are many ways to project the images to each eye, such as through anaglyphs (“filtering” the images with the use of blue-red display and glasses) or polarization (“filtering” the images with the use of vertical and horizontal polarization display and glasses) or other methods that are used in modern stereomonitors. This pair of images is called a stereogram or stereopair. The science or art that deals with the use of binocular vision for the observation and production of ste-reograms is called stereoscopy.

The average distance between the human eyes is about 65 millimeters. The human brain can differentiate (perceive parallax) between two objects when they are at a minimum angular distance of about 15 inches. With the use of simple triangulation, we can estimate the minimum depth distance judgment according to the distance of an object from the human eyes. The further the object is from our eyes, the more difficult it gets for us to judge its depth distance. If, however, we can somehow increase the distance between our eyes, then we would get better depth perception, and thus, the distance of the object could be better estimated (Figure 1).

The distance between the two images of the object is called a baseline, and the larger it is, the less the error of the object positioning and thus the more accurately the depth distance is calculated (assuming that the other parameters are the same).

As photogrammetry concerns itself with the geometric measurements of objects in analog or digital images, the stereoscopic measurement principle is directly applied. With the production of images of the same object from two different places in space (stereoscopy), one could make measurements of the object's depth distance. This way, the recording and measurement of the object are done in the three-dimensional (3D) world and not just in the 2D world of a piece of paper or a computer monitor.

...