Principles of Photogrammetry

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Principles of Photogrammetry

• GRG S362K
• Peter H. Dana

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Photogrammetry

• Science (or art) of deducing the physical
  dimensions of objects from measurements on
  photographs 
• Mapping the earth or other bodies in the solar
  system 
• Sometimes used to indirectly measure the geometry
  of buildings, dams, archeological sites using
  photographs. 
• Sometimes the same principles are applied to
  digital imagery from satellite-based RS
  platforms.

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Single Photograph Approximations

• Scale of Photograph
• Height of Objects
• Length of Objects
• Areas of Regions
• Perimeter of Regions
• Tone of Objects

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Multiple Photograph Measurements

• Object location
• Object height
• Contour Maps
• Orthophotoquads
• Digital Elevation Models
• Slope and Aspect from DEMs

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Flightline

• Path of aircraft
• Nadir Line
• Directly beneath aircraft in flight
• Intervalometer (timed sequence)
• 60 - 80 overlap
• 2 or more views of one object
• 20 - 30 sidelap

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Aerial Photographs

• Fiducial Marks
• Four corners and four mid-points
• Principal Points
• Nadir and Flightline
• Conjugate Principal Points
• Principal point for A on B (and B on A)

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Principal Point
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Conjugate Principal Point (Jensen)
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Focal Length

• Focal Length?
• Distance from the lens at which parallel light
  rays are focused to a point.

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Scale of Single Photograph

• Scale Photograph Size of Object / Real World
  Size of Object
• 1 inch football field 100 yards scale
  11,200
• Scale camera focal length / Height of Camera
• 12 inch focal length lens / 1,200 foot flight
  altitude scale 11,200

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Height from Displacement

• Height of Object from Relief Displacement
• Distance of top from principal point r
• Relief displacement (top to bottom on plane) d
• Height of Object h
• Height of Camera H
• h dH/r
• Problems
• Height of camera
• Errors in measurements on photograph are
  magnified

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Height from Shadow

• Suns elevation angle (a function of time of day
  and date) a
• Length of Shadow L
• h L tan(a)
• Problems
• Uneven ground, leaning objects, shadow not from
  top, ground cover showing shadow.

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Parallax

• Apparent change in position caused by change in
  viewing angle
• Distance between an object on two photographs and
  the distance between the principal points is a
  measure of the height of the object.
• A mark can be superimposed on two photographs
  such that it will touch the ground in the fused
  stereo view at a specific height. This allows the
  tracing of contour lines on a paper map or the
  digitization of contour lines into a computer

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Parallax (Robinson)
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Parallax (Muehrcke)
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References

• Lillesand and Kiefer, 2000. Remote Sensing and
  Image Interpretation, 4th edition. New York John
  Wiley and Sons.
• Muehrcke, Phillip C., J. O. Muehrcke, and A. J.
  Kimerling. 2001. Map Use Reading, Analysis,
  Interpretation. Fourth Edition. Madison, WI JP
  Publications.
• Robinson, A. H., J. L. Morrison, P, C, Muehrcke,
  A. J. Kimerling, and S. C. Guptill. 1995.
  Elements of Cartography. Sixth Ed. New York John
  Wiley sons