7.3 Stereoscopy

7.3 Stereoscopy

A pair of stereoscopic photographs or images can be viewed stereoscopically by looking at the left image with the left eye and the right image with the right eye. This is called stereoscopy. Stereoscopy is based on Porro-Koppe's Principle that the same light path will be generated through an optical system if a light source is projected onto the image taken by an optical system. The principle will be realized in a stereo model if a pair of stereoscopic images are reconstructed using the relative location or tilt at the time the photography was taken. Such an adjustment is called relative orientation in photogrammetric terms. The eye-base and the photo-base must be parallel in order to view at a stereoscopic model, as shown in Figure 7.3.1.

Usually a stereoscope is used for image interpretation. There are several types of stereoscope, for example, portable lens stereoscope, stereo mirror scope (see Figure 7.3.2), stereo zoom transfer scope etc.

The process of stereoscopy for aerial photographs is as follows. At first the center of both aerial photographs, called the principal point, should be marked. Secondly the principal point of the right image should be plotted in its position on the left image. At the same time the principal point of the left image should be also plotted on the right image. These principal points and transferred points should be aligned along a straight line, called the base line, with an appropriate separation (normally 25-30 cm in the case of a stereo mirror scope ) as shown in Figure 7.3.3. By viewing through the binoculars a stereoscopic model can now be seen.

The advantage of stereoscopy is the ability to extract three dimensional information, for example, classification between tall trees and low trees, terrestrial features such as height of terraces, slope gradient, detailed geomorphology in flood plains, dip of geological layers and so on.

The principle of height measurement by stereoscopic vision is based on the use of parallax, which corresponds to the distance between image points, of the same object on the ground, on the left and right image. The height difference can be computed if the parallax difference is measured between two points of different height, using a parallax bar, as shown in Figure 7.3.3.