Stereovision and Photo Stereovision and Photo InterpretationInterpretation

3-dimensional viewing of aerial photos

How does stereo vision work?How does stereo vision work?

Each eye views the same feature from a difference Each eye views the same feature from a difference angle/perspective and the brain fuses the images into angle/perspective and the brain fuses the images into a single coherent imagea single coherent image

The greater the distance between viewing stations The greater the distance between viewing stations (eye base or photo base) the better the depth (eye base or photo base) the better the depth perceptionperception

There is vertical exaggeration when viewing aerial There is vertical exaggeration when viewing aerial photos with a stereoscope photos with a stereoscope

With a stereoscope the relationship between With a stereoscope the relationship between accommodation (ability of eyes to change focus based accommodation (ability of eyes to change focus based on distance to an object) and convergence is on distance to an object) and convergence is unnaturalunnatural

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

stereostereo

Tools for stereoscopic Tools for stereoscopic interpretationinterpretation

Aerial photo stereo pairsAerial photo stereo pairs StereoscopeStereoscope Transparency to protect photosTransparency to protect photos Pens or other markersPens or other markers Pin to prick holes to mark principle pointsPin to prick holes to mark principle points Tape or weights to hold photos to tableTape or weights to hold photos to table Good lighting Good lighting Interpretation key to aid in identifying Interpretation key to aid in identifying

featuresfeatures i.e., i.e., http://www.treesearch.fs.fed.us/pubs/6451http://www.treesearch.fs.fed.us/pubs/6451

Measuring tools (parallax bar, ruler)Measuring tools (parallax bar, ruler)

StereoscopesStereoscopes

Pocket stereoscopesPocket stereoscopes Handy for use in the fieldHandy for use in the field Simple and inexpensive (~$5 – 100)Simple and inexpensive (~$5 – 100) Cumbersome when working with large format Cumbersome when working with large format

photographyphotography Mirror stereoscopesMirror stereoscopes

Usually large enough to easily view 23cm photosUsually large enough to easily view 23cm photos Come with additional lenses for magnified viewingCome with additional lenses for magnified viewing Quality ranges from simple (~$140) to precise Quality ranges from simple (~$140) to precise

(~$3,000)(~$3,000) Mirrors can be very delicate and require special Mirrors can be very delicate and require special

carecare

Mirror stereoscope Pocket

stereoscope

Orientation of stereo photosOrientation of stereo photos

Verify photos are consecutively numbered and Verify photos are consecutively numbered and there is at sufficient forward overlapthere is at sufficient forward overlap

Orient photos so shadows fall toward the observer Orient photos so shadows fall toward the observer to reduce pseudoscopic effectto reduce pseudoscopic effect

Locate the principal point and the conjugate Locate the principal point and the conjugate principal point for each photoprincipal point for each photo

Align the two photos using a ruler so the 4 points Align the two photos using a ruler so the 4 points are on the same lineare on the same line

Tape or put weights on the photos to hold them in Tape or put weights on the photos to hold them in placeplace

Overlay with transparency to protect photos and Overlay with transparency to protect photos and mark fiducial marks and photo reference mark fiducial marks and photo reference

Viewing stereo photos on a Viewing stereo photos on a computercomputer

Photos must be properly oriented before Photos must be properly oriented before viewing and this requires special software viewing and this requires special software such as ERDAS Leica Photogrammetry Suitesuch as ERDAS Leica Photogrammetry Suite

Viewing the georeferenced stereo pairs Viewing the georeferenced stereo pairs requires special software such as Stereo requires special software such as Stereo Analyst Analyst

Viewing on the screen can be done using:Viewing on the screen can be done using: Red and blue anaglyph glassesRed and blue anaglyph glasses LCD shutter glasses (requires special graphics LCD shutter glasses (requires special graphics

card)card) Polarized screen and glassesPolarized screen and glasses