Short note: Stereoscopy, aerial mapping

Principle of stereoscopy

Stereoscopy is the ability to see in three dimensions. A person's two eyes are about 65 mm apart (the eye base). Each eye views an object from a slightly different angle, so the two retinal images differ; the brain fuses them into a 3D impression and judges depth from the parallax between the images.

In photogrammetry, two overlapping photographs of the same area taken from different camera stations (air base $B$) serve as the "two eyes". When the left photo is seen by the left eye and the right photo by the right eye (using a lens or mirror stereoscope, or anaglyph or polarised displays), the overlap is seen as a three-dimensional model. Height differences are measured from the differences in parallax: $h = \dfrac{H\,\Delta p}{p + \Delta p}$, where $p$ is the absolute stereoscopic parallax of the lower (base) point, $\Delta p$ the difference in parallax, and $H$ the flying height above the base point.

   O1 <------ B ------> O2     two exposures
    \                 /
     \               /
      \             /
       [ overlap ]  -> 3D model

Importance of aerial mapping

• Quick preparation of maps of large and inaccessible areas.
• Uniform accuracy and a permanent record of the ground at the time of the flight.
• Cheaper and faster than ground survey for large areas.
• Gives contours, spot heights, planimetric details and orthophotos for the planning and design of roads, dams and cities.
• Allows maps to be updated and changes to be monitored (floods, landslides, urban growth).