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Understanding Stereovision in Aerial Photo Interpretation

Learn how stereo vision works in interpreting aerial photos, tools needed, types of stereoscopes, orientation techniques, and viewing on a computer using specialized software and glasses.

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Understanding Stereovision in Aerial Photo Interpretation

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  1. Stereovision and Photo Interpretation 3-dimensional viewing of aerial photos

  2. How does stereo vision work? • Each eye views the same feature from a difference angle/perspective and the brain fuses the images into a single coherent image • The greater the distance between viewing stations (eye base or photo base) the better the depth perception • There is vertical exaggeration when viewing aerial photos with a stereoscope • With a stereoscope the relationship between accommodation (ability of eyes to change focus based on distance to an object) and convergence is unnatural • Not everyone can see in stereo • There are exercises to develop the ability to see in stereo

  3. Tools for stereoscopic interpretation • Aerial photo stereo pairs • Stereoscope • Transparency to protect photos • Pens or other markers • Pin to prick holes to mark principle points • Tape or weights to hold photos to table • Good lighting • Interpretation key to aid in identifying features • i.e., http://www.treesearch.fs.fed.us/pubs/6451 • Measuring tools (parallax bar, ruler)

  4. Stereoscopes • Pocket stereoscopes • Handy for use in the field • Simple and inexpensive (~$5 – 100) • Cumbersome when working with large format photography • Mirror stereoscopes • Usually large enough to easily view 23cm photos • Come with additional lenses for magnified viewing • Quality ranges from simple (~$140) to precise (~$3,000) • Mirrors can be very delicate and require special care

  5. Mirror stereoscope Pocket stereoscope

  6. Orientation of stereo photos • Verify photos are consecutively numbered and there is at sufficient forward overlap • Orient photos so shadows fall toward the observer to reduce pseudoscopic effect • Locate the principal point and the conjugate principal point for each photo • Align the two photos using a ruler so the 4 points are on the same line • Tape or put weights on the photos to hold them in place • Overlay with transparency to protect photos and mark fiducial marks and photo reference

  7. Viewing stereo photos on a computer • Photos must be properly oriented before viewing and this requires special software such as ERDAS Leica Photogrammetry Suite • Viewing the georeferenced stereo pairs requires special software such as Stereo Analyst • Viewing on the screen can be done using: • Red and blue anaglyph glasses • LCD shutter glasses (requires special graphics card) • Polarized screen and glasses

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