Non-Conventional Aerial Photography: Continuous Strip Camera and Panoramic Camera (Especially for GATE-Geospatial 2022)

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Photographic Camera vs. Non-Conventional Aerial Camera

  • An aerial camera whose geometric characteristics are exactly known from calibration and which can produce a single perspective (or central) projection in the resulting photograph. This type of aerial camera is more properly called the photogrammetric camera and is usually employed for conventional aerial photography for topographic mapping.
  • Non-conventional aerial cameras are less suited for topographic mapping by photogrammetric methods but may be useful as a source of descriptive information for geographical applications. Worth mentioning are the two non-conventional photographic systems:
    • The Continuous Strip Camera (or Sonne Camera)
    • The Panoramic Camera

Continuous Strip Camera

  • This camera is a type of reconnaissance camera for military purposes which has been designed for low-altitude, high-speed photography. It exposes a continuous photograph of the terrain from the aircraft by passing the film over a stationary slit in the focal plane of the lens at the speed of the aircraft flight. This compensates for the image motion relative to the terrain.
  • There is no shutter in the camera but the aperture of the lens and the size of the slit can be varied. The slit is usually very small so that only a narrow ‘ribbon’ or ‘strip’ of the terrain is exposed on the film. As the aircraft moves forward, a long continuous photograph is produced by successive integration of these narrow ribbons, hence the name of the camera. It is obvious that with this type of camera the single-point perspective projection is destroyed as a result of the moving film.
  • It is not possible to produce stereo pairs of photographs using a single-lens cone for the camera. If stereo-photographs are required, a dual-lens cone with one lens displaced forwards and the other backwards has to be employed in order to create a stereo base in the direction of the flight. But because of the extended stereo base, the stereo model cannot be obtained without a specially constructed stereoscope as an aid. It is particularly important to eliminate tilts at the time of photography; otherwise, gaps will occur in the resultant coverage.

Panoramic Camera

The panoramic camera is another photographic system for military aerial reconnaissance. Basically, it consists of a moving lens and a slit which sweeps out a cylindrical path on to a film which may be stationary or moving at the same speed as the aircraft (i.e.. with Image Motion Compensation) .

Panoramic Camera
  • The points on the ground are no longer imaged at their correct perspective positions on the film but become displaced according to the cylindrical shape of the negative film surface and the scanning action of the lens. Thus, in the above figure, the ground point P is imaged as P ″ with curved coordinate distances of Xp ″ and Yp ″ from the centre of the cylindrical film surface which may be compared with the corresponding perspective position and straight line coordinate distances on a plane film surface (i.e. P ′ , Xp ′ and Yp ′ ) .
  • This type of positional displacement is known as panoramic distortion and an example of recording a unit grid on flat ground (Figure a) with panoramic distortion is shown in Figure b. In addition, the forward motion of the aircraft during the time of scanning also modifies the position of points owing to panoramic distortion, thus giving rise to a sweep positional distortion which affects the Y-centre line as shown by the dotted S-curves in Figure b.
  • In the case where Image Motion Compensation is incorporated, a third distortion also occurs owing to the lateral movements of the lens or focal plane (the film) . This modifies the position of points owing to both panoramic distortion and sweep distortion in the X-direction but not in the Y-direction.
  • The overall effect of all these distortions in modifying the unit grid on flat ground is shown in Figure c. It is obvious that for the panoramic photographic system, the single-point perspective projection is not preserved and that the internal geometric characteristics of the system are difficult to determine exactly.
Distortions for Panoramic Photography

Distortions for panoramic photography (after Hovey, 1965) : (a) Original grid on the ground (b) Panoramic distortion effect (1) , sweep positional distortion effect (2) , and image motion compensation effect (3) (c) Pattern of the grid in (a) with all distortions combined.

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