Remote Sensing
Geometry of Aerial Photographs
1. Geometry of Vertical Aerial
Photograph
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Oblique photographs
- Cameras oriented toward the side of the aircraft
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Vertical photographs
- camera aimed directly at the ground surface
from above
- difficult to recognize ground features but
measurements can be made
Geometry of Aerial Photo
Black and White Aerial Photographs
Color Aerial Photographs
NIR Aerial Photographs
Photo taken as part of the NAPP mission in color
IR at a scale of 1:40,000. (1”=3,333’). This
image has been enlarged by x factor.
1. Geometry of Vertical Aerial
Photograph
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Photogrammetry
- science of making accurate measurements from
aerial photographs
2. Basic Elements
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Fiducial marks
Principle point
Ground nadir
Photographic nadir
Isocenter
Frame Number
Date of
Photography
Nadir Point
Principal Point
Fiducial Marks
Mission ID
2. Basic elements
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Fiducial marks
- at the edges and corners recorded during
exposure
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Principle point
- intersection of lines connecting opposite pairs of
fiducial marks
Basic Elements
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Ground nadir
- point on the ground vertically beneath the
center of the camera lens during exposure
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Photographic nadir
- intersection of the photograph and the vertical
line that intersects the ground nadir and the
center of the lens
Basic Elements
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Isocenter
- the focus of tilt
- on a true vertical photo, the isocenter, the
principle point, and the photographic nadir
coincide
Vertical Aerial Photography
Vertical Aerial Photography
Most common type aerial photography
► Camera axis as vertical as possible
► Generally a tilt or error introduced in photography
of 1 – 3 degrees
► Small amount can be easily adjusted
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Geometric Errors of Vertical Aerial
Photography
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Optical distortion
- caused by camera problems
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 than the normal scale
- Roll, Crab, Pitch
Tilt
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Roll distortion
- about its flight axis
- roll compensation
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Crab distortion
- caused by deflection of aircraft due to crosswind
- corrections: on the plane or by computer
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Pitch distortion
- result in local scale change
- can be ignored in most analyses
Tilt
3. Relief Displacement
The direction of relief displacement is radial from
nadir
► It increases with (1) increasing height of the
feature and (2) the distance from nadir
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Relief Displacement
4. Multiple Photographs
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Forward overlap: 50-60% along flight line; sidelap: 5-15%
Stereoscopic Pairs
Flight line # 1
Flight line #
2
Pre-Marked Points
Panels (targets) are placed at control point
locations prior to the flight so that they will appear
on the photography
► Panels size depends on the scale of the
photography
► Painted on hard surfaces
► Made of plastic sheeting, cloth, or wood on grass
or soil surfaces
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Multiple Photos
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Stereoscopic parallax
- difference in appearances of objects due to
change in perspectives
- it can be measured to compute the elevations
of terrain
Photogrammetry
Measurement
Orthophotographs
5. Orthophotographs
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Aerial photographs without geometric errors
introduced by tilt or relief displacement
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Orthophotomaps and digital orthophoto Quarter
Quad (DOQQ)
- show correct planimetric position and consistent
scale
Readings
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Chapter 3
Multiple Photos
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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