Geometry of Aerial Photograph

1. Geometry of the vertical aerial photograph
   Oblique photographs
        -cameras oriented toward the side of the aircraft

   Vertical photographs
        - camera aimed directly at the ground surface from above
        - difficult to recognize ground features but measurements can be made

   Photogrammetry
        -science of making accurate measurements from aerial
            photographs

2. Basic elements
        Fiducial marks:
        - at the edges and corners recorded during exposure

        Principle point
        - intersection of lines connecting opposite pairs of fiducial
            marks

        Ground nadir
        - point on the ground vertically beneath the center of the
            camera lens during exposure

        Photographic nadir:
        - intersection of the photograph and the vertical line that
            intersects the ground nadir and the center of the lens

        Isocenter:
        - the focus of tilt
        - on a true vertical photo, the isocenter, the principle
            point, and the photographic nadir coincide
 

Geometric Errors of the Vertical Aerial Photography

1. Optical distortion
        - caused by camera problems

2. Tilt
Caused by displacement of the focal plane from a truly horizontal position
by aircraft motion (attitude)

        - image areas on the upper side of the tilt depict ground
            features in smaller scale than the normal scale

   Roll distortion
        - about its flight axis
        - roll compensation

   Crab distortion
        - caused by deflection of aircraft due to crosswind
        - corrections: on the plane or by computer

   Pitch distortion
        - result in local scale change
        - can be ignored in most analyses

3. Relief displacement
        - the direction of relief displacement is radial from nadir
        - it increases with increasing height of the feature
          and the distance from nadir

4. Multiple photographs
        Forward overlap: 50-60%; along flight line
        sidelap: 5-15%

   Stereoscopic parallax:
        - difference in appearances of objects due to change in
            perspectives
        - it can be measured to compute the elevations of terrain

   Conjugate principle
        - the image centers of preceding and succeeding photographs
        - lines connecting the conjugate principle points define
            flight axis
        - parallax occurs parallel to the line of flight

5. Orthophotos
        - aerial photographs without geometric errors introduced by
            tilt or relief displacement
        - differential rectification using orthophotoscope

        Orthophotomaps and digital orthophoto Quarter Quad (DOQQ)
        - show correct planimetric position and consistent scale

6. Reading: Chpt 3
 
 
 
 
 

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