Stereoscopic Image Generation, Interpretation and Limitations: 3 − D Airphoto Interpretation (Especially for GATE-Geospatial 2022)

Doorsteptutor material for CBSE/Class-7 is prepared by world's top subject experts: get questions, notes, tests, video lectures and more- for all subjects of CBSE/Class-7.

Examrace Books on Mapping, GIS, and Remote Sensing prepares you throughly for a wide range of practical applications.

Because of the overlap between successive airphotos along a flight, it is possible to view airphotos stereoscopically, i.e.. in three dimensions. However, stereoscopic viewing is limited to the area of overlap between the images.

Principle of Stereoscopic Viewing

Stereoscopic viewing is based on binocular vision. Each of our eyes sees a scene from a slightly different perspective. Our brains reconstruct the two images recorded by our eyes into a three-dimensional view of the scene. The same thing is possible with airphotos (or other images) provided that when we view the airphotos, each eye is focused on a single image.

There are several methods that can be used to ensure that each eye sees only one of a pair of images. Early 3-D movies relied on the use of anaglyphs. To see the movie in 3-D, the audience was required to wear glasses with the red and green lens. The coloured lenses filter out different colours, so each eye sees a different image which the brain reconstructs into a perspective view. Polarized light and projectors operate in a similar way. By changing the direction of polarization, each eye views a different image.

Stereoscopic Viewing in Airphoto Interpretation

In Airphoto interpretation, stereoscopic viewing is usually assisted using pocket of mirror stereoscopes. Both operate on the same principle. Mirror stereoscopes have the advantage of being able to view larger images than is possible with a pocket stereoscope which is limited by the approximately 5 cm distance between our eyes. We look at a pair of overlapping airphotos through lenses that force each eye to see only one of the pair of the photo. Once again, our brain reconstructs the three-dimensional view from the pair of images.

Pocket Stereoscope

Pocket Stereoscope

Mirror Stereoscope

Mirror Stereoscope

Parallax Angle

Depth perception is a function of the parallax angle, which is the angle between the eyes and an object in a pair of stereo images. The parallax angle decreases with distance from the object. Because of radial displacement of objects in the image, the top of an object appears to be at a different depth than the bottom of an object.

Problems with Stereoscopic Vision

Parallax Angle

Parallax Angle

Pseudoscopic Vision

In setting up airphotos for stereoscopic viewing, care must be taken to avoid pseudoscopic vision. Pseudoscopic vision can occur in two ways:

  • If the order of the airphotos is reversed.
  • If the shadows in the image point away from the observer.

Both conditions will cause the 3-D image to appear to be inverted.

Pseudoscopic Vision

Vertical Exaggeration

A final problem with three-dimensional viewing of airphotos is a vertical exaggeration. Objects in the image appear to be taller than in reality and slopes appear to be steeper. This exaggeration can sometimes aid in interpretation but is somewhat disorienting to inexperience viewers. Vertical exaggeration occurs because of the difference in geometry when taking the airphotos and when viewing the airphotos. Vertical exaggeration varies with camera focal length and % overlap between successive images. Vertical exaggeration can be calculated as:

, where: B is the air base; H is the height of the aircraft above the ground; b is the eye base (approximately 6 cm) and h is the distance from the eye at which the stereo model is perceived (approximately 45 cm)

Vertical Exaggeration

Developed by: