Handling of Imagery: Transparencies & Stereo Viewing (Especially for GATE-Geospatial 2022)

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Using Transparencies

Although a good deal of photo interpretation is still done using paper prints, the use of diapositive transparencies are increasing. Transparencies can be used either as single frames or as a roll. Care should be taken when handling transparencies so that they are not marred. An orderly procedure for the handling of either prints or transparencies should be developed and adhered to in any interpretation project. Airphotos are typically numbered with flight name and/or frame number and should be kept in order as far as practical. Different dates and flight lines should be kept separate, etc. Anytime transparencies are used surfaces should be as clean as possible and the interpreter should either wear cotton gloves or be sure not to touch the emulsion surface as skin oils can cause image deterioration.

Stereo Viewing

Binocular vision is natural to all of ′ us, but to the trained interpreter the ability to perceive stereo is an incredibly asset. Stereo viewing will be covered in detail later, but suffice it to say that viewing high-quality stereo aerial photography through a mirror stereoscope is like seeing in another dimension. Although the identification and interpretation of many landscapes can be accomplished with mono, stereo is required for certain types of studies. The following are some tips for using stereo effectively.

Basics for Stereo-Viewing

  1. Make certain that the photos are properly aligned, preferably with the shadows falling toward the viewer.
  2. Keep the eye base and the long axis of the stereoscope parallel to the flight line.
  3. Maintain an even glare-free illumination on the prints or transparencies.
  4. Arrange for comfortable sitting and enough illumination.
  5. Keep the lenses of the stereoscope clean, properly focused and separated to your interpupillary distance.
  6. The novice interpreter should not work with stereo more than 30 minutes out of an hour period.

Trouble Shooting Stereo

  1. Your eyes may be of unequal strength. If you normally wear glasses for reading or close-up work, you should also wear glasses when using the stereoscope.
  2. Poorly illumination, misaligned prints or uncomfortable viewing positions may result in eye fatigue.
  3. Illness or severe emotional distress may create sensations of dizziness in one using a stereoscope.
  4. Reversal of prints may cause pseudo-stereo. A similar problem may occur if prints are aligned with the shadows falling away from rather than towards the interpreter.
  5. Objects that change positions between exposures cannot be viewed in stereo.
  6. In areas of steep topography, scale differences in adjacent photographs may make it difficult to obtain a three-dimensional image.
  7. Dark shadows or clouds may prohibit stereo viewing of an area by obscuring an object on one photo.

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