Photogrammetry . Lecture 7

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• Lecture 7 Content

• Stereo plotters

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• Stereo plotters
• The principle is that each aerial photograph is
  formed by a bundle of light rays which originate
  at the terrain surface and pass through the lens
  of the aerial camera to form an image on the film
• Therefore, each terrain point in the stereo model
  is considered to be the origin of two rays (one
  on each photograph)

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• The stereo model is constructed by using the two
  photographs in two projectors but the original
  separation between exposure stations (the air
  base, B) is reduced from its original value. For
  example 1000m air base is reduce to 200mm
  instrument base
• If the original situation are the same then, the
  intersections of bundles of rays will now form a
  three dimensional model of the terrain at a scale
  of bB (where b is the instrument base set for
  the two projectors)

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At the time of flight (air base B)
B
O2
O1
Terrain
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• In order to form the stereo model, it is
  necessary to exactly reconstruct the bundles of
  rays which formed the original photographs and to
  reestablish their positions relative to the datum
  of terrain coordinate system
• Stereo plotting instruments enables the formation
  of such stereo models

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Traditional stereo plotter

• Instruments of this type is not produced any more

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• The sophisticated construction of the modern
  stereoplotters does not make the principle so
  obviously visible
• The main function of stereoplotters is to provide
  accurate formation of a three dimensional stereo
  model at a given scale and to enable the
  compilation of a map

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• Stereoplotters embodies three main components
• Projection system which creates the model
• Observation system which makes it possible for
  the operator to view the model
• Measuring and tracking device which enables
  scanning and measuring of the stereo model

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• Stereoplotters are classified into two main
  categories
• Analogue stereoplotters which create a stereo
  model by intersection of light rays (projection
  through lenses) or by intersection of mechanical
  rods, and
• Analytical plotters which establish the
  projection relationship by mathematical formulae
  and employ the computer to generate the model.
• Analogue instruments are not produced any more
  but are still used by some professionals

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• Analogue Stereoplotters
• Optical Stereoplotters
• Makes use of light and optics to generate the
  stereo model
• Employ red and green anaglyph filters to obtain
  image separation for stereoscopic viewing of the
  model with anaglyph spectacles
• Plotting is carried out using a single measuring
  mark which can be moved around the model and
  raised or lowered on a tracing stand available in
  the instrument that is used to measure elevation

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Zeiss DP1 analogue stereoplotter
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• Mechanical stereoplotters
• The lens of the camera is replaced by a universal
  joint or cardan
• Two rays are represented by metal rods called
  space rods
• Manufacturers are Zeiss, Kern, Galileo, etc.

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WILD B8S analogue stereoplotter
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• Analytical plotters
• Analytical plotters makes use of
  computer-assisted computations
• The observation systems are similar to those
  employed for mechanical analogue stereoplotters
• Film and paper prints are used as photographic
  input materials which are scanned and fed to the
  computer model

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• Analytical plotters solve the relationship
  between photograph and terrain by applying
  mathematical formulae defined by the collinear
  and coplanar equations
• The computer performs all computations (that is
  transforming the photo-coordinates to the terrain
  coordinates system etc.) and controls the
  operation of the instrument
• Analytical plotter is provided with interactive
  editing capabilities to enable the comfortable
  compilation of maps

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• Additional facilities may include
• The possibility of driving the floating mark to
  any specified terrain point
• The application of elevation measurements using
  image correlators to generate elevation
  information
• Superimposition of all measured lines onto the
  photo image viewed by the operator
• Digital aerial photographs may be directly
  displayed and observed on a graphic screen

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Traditional photogrammetric softcopy workstation
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Photogrammetric softcopy workstation (Geomatics
Program, TSU)
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• Stereoscopic parallax
• Parallax is the distance between images of the
  same object on the two photographs forming a
  stereopair

px
A2
z
py
y
A1
x
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• Parallax is the apparent change in position of an
  object, in relation to a reference object, due to
  a change of observation
• Stereoscopic parallax enables perception of
  height differences
• Parallax disturbs stereoscopic viewing and must
  be eliminated as far as possible before
  stereoscopic observations starts

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X-Parallax

• The apparent displacement of an object caused by
  the shift in the position of observation

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X-Parallax
Change in position of rooftop termed stereoscopic
parallax

• Parallax of a point is directly related to its
  elevation
• Parallax is greater for high points than low
  points

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• The End