Stereovision and Photo Interpretation 3-dimensional viewing of aerial photos

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 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 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 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 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 Not everyone can see in stereo There are exercises to develop the ability to see in stereo There are exercises to develop the ability to see in stereo

Tools for stereoscopic interpretation Aerial photo stereo pairs Aerial photo stereo pairs Stereoscope Stereoscope Transparency to protect photos Transparency to protect photos Pens or other markers Pens or other markers Pin to prick holes to mark principle points Pin to prick holes to mark principle points Tape or weights to hold photos to table Tape or weights to hold photos to table Good lighting Good lighting Interpretation key to aid in identifying features Interpretation key to aid in identifying features i.e., i.e., Measuring tools (parallax bar, ruler) Measuring tools (parallax bar, ruler)

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

Mirror stereoscope Pocket stereoscope

Orientation of stereo photos Verify photos are consecutively numbered and there is at sufficient forward overlap Verify photos are consecutively numbered and there is at sufficient forward overlap Orient photos so shadows fall toward the observer to reduce pseudoscopic effect Orient photos so shadows fall toward the observer to reduce pseudoscopic effect Locate the principal point and the conjugate principal point for each photo 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 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 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 Overlay with transparency to protect photos and mark fiducial marks and photo reference

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