Stereoviewing And Photogrammetric Analysis

In Chapter 3, the fundamentals and benefits of stereoscopic analysis already have been introduced. However, stereo-scopy also might be useful for visual interpretation disregarding quantitative measurements—many aspects of a scene may reveal themselves to the observer more clearly or exclusively in 3D view. Generally, stereoscopic viewing may be achieved either with or without optical aids, including the following approaches (Jensen, 2007).

• Place the images in reading distance (left image to the left and right image to the right side), relax eyes as if looking at infinity until the two images fuse into an un-focussed third 3D image in the middle, and focus back on this stereomodel (try with Figs. 2-9 and 3-14).

• Place the images in reading distance (left image to the right and right image to the left side), let the lines of sight cross until the two images fuse into a third 3D image in the middle. Note: using this method on the stereo-images in this book will cause relief inversion.

• Print one image in red and the other in blue, green, or cyan on top of each other to create anaglyphs, use corresponding anaglyph lenses to view.

• Arrange the images under a lens or mirror stereoscope (seeFig. 2-10).

• Use stereoviewing software and hardware, e.g. electronic shutter lenses.

The first two methods are a matter of practice; some people can do this easily, but most cannot without causing

FIGURE 11-17 Supervised maximum likelihood classification of the image shown in Figure 11-12A; image processing by IM. (A) Classification of original RGB image. (B) Classification of composite of green image band and ratio image of green and blue image band shown in Figure 11-12B. Dark green = pine trees; medium green = grasses and shrubs; light green = bare soil.

FIGURE 11-17 Supervised maximum likelihood classification of the image shown in Figure 11-12A; image processing by IM. (A) Classification of original RGB image. (B) Classification of composite of green image band and ratio image of green and blue image band shown in Figure 11-12B. Dark green = pine trees; medium green = grasses and shrubs; light green = bare soil.

considerable eye strain. These methods are only useful for an aid-free, quick appraisal of a stereoscene, and zooming in on details is obviously not feasible. Anaglyphs are easy to realize and work well for most people, but they are color-distorting, a real nuisance to the human brain and not recommended for serious work. Stereoscopes and their digital realizations are clearly the best, but also most expensive option.

Irrespective of the viewing method, the images need to be orientated so they reflect their relative position at the moment of exposure. For SFAP images this is usually not as simple as placing them next to each other in parallel, as SFAP surveys rarely result in regularly aligned flightlines with consistent overlaps (see Fig. 3-10). In photogrammetry software, the relative orientation is computed via GCPs and tie points. The manual method for reconstructing the flightline between the two exposures, which needs to be aligned to the eye base of the observer, is illustrated by Figure 11-18. Mark (or for quick casual viewing roughly identify and memorize) the center point (principal point, o) in each image and also its corresponding point (o') in the other. Then lay out both images with the center points at the outside and their corresponding points at the inside so all fall on one straight horizontal line. There are two ways to do this (the whole arrangement may be turned by 180°); in case of sunlit scenes it is best to choose the variant where the shadows are cast toward the lower right rather than the upper left in order not to confuse the brain (see Fig. 5-3). The ideal spacing between the two images depends on the viewing method and the observer's liking.

In the following, three stereoviewing methods of practical use for SFAP are briefly described; the orientation technique presented above works for all of them.

100 Photography Tips

100 Photography Tips

To begin with your career in photography at the right path, you need to gather more information about it first. Gathering information would provide you guidance on the right steps that you need to take. Researching can be done through the internet, talking to professional photographers, as well as reading some books about the subject. Get all the tips from the pros within this photography ebook.

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