3.2 Aerial Photography
In texts on aerial photogrammetry or photo-interpretation, various types of aerial photographic cameras are discussed in detail (e.g. Lillesand and Kiefer, 1994; Paine, 1981). The implications of flight height, photographical orientation, and view angle on aerial photographic products are briefly discussed here.
For a given focal length of an aerial camera, the higher the camera is, the larger the area each aerial photo can cover. Obviously, the scale of aerial photographs taken at higher altitudes will be smaller than those taken at lower altitudes.
However, photographs taken at higher altitudes will be severely affected by the atmosphere. This is particularly true when films sensitive to shorter wavelengths are used. Thus, ultraviolet and blue should be avoided at higher altitudes. Instead, CIR or BWIR is more suitable.
Two types of camera orientations maybe used: vertical and oblique(slant) (Figure 3.13). Oblique allows one to take pictures of a large area while vertical allows for less distortion in photo scale.
Figure 3.13. Vertical and slant aerial photography
View angle is normally determined by the focal length and the frame size of a film. For a camera, the frame is fixed, therefore the ground coverage is determined by the altitude and the camera viewing angle (Figure 3.14)
f1 > f2 > f3
a1 < a2 < a3
Figure 3.14. Viewing angle determined by the focal length
In normal cameras Aerial Camera Normal lens 50 mm 300 mm Wide angle 28 mm 150 mm Fisheye lens 7 mm 88 mm
Obviously, wide angles allow a larger area to be photographed.
Spatial resolution of aerial photographs is largely dependent on the following factors:
ï Lens resolution
ï optical quality
ï Film resolution
ï Film flatness -normally not a problem
ï Atmospheric conditions -changes all the time
ï Aircraft vibration and motion -random
Film resolution depends mainly on granularity.
There is a standard definition of photographic resolution is the maximum number of line-pairs per mm that can be distinguished on a film when taken from a resolution target (Figure 3.15).
If the scale of an aerial photograph is known, we can convert the photographic resolution (rs) to ground resolution.
Figure 3.15. Resolving power test chart (from Lillesand and Kiefer, 1994).
A photograph may have a small coverage if it is taken either at a low flight height or with a narrower viewing angle.
The advantages of photographs with small coverages are that they provide more detail, and less distortion and displacement. It is easier to analyze a photograph with a small coverage because similar target will have less distortion from the center to the edge of the photograph, and from one photograph to the other.
The disadvantage of photographs with small coverages is that it needs more flight time to cover an area and thus the cost will be higher. Moreover, mosaicing may cause more distortion.
A large coverage can be obtained by taking the photograph from a higher altitude or using a wider angle. The quality of photographs with a large coverage is likely to have poorer photographic resolution due to larger viewing angle and likely stronger atmospheric effect. The advantages are that a large coverage is simultanuously obtained, requires less geometric mosaicing, and costs less.
The disadvantages are that it is difficult to analyze targets in detail and that target is severely distorted.
Essentially, the size of photo coverage is related to the scale of the raw aerial photographs. Choosing photographs with a large coverage or a small one should be based on the following:
ï budget at hand
ï equipment available
The following are some of the advantages/disadvantages of aerial photography in comparison with other types of data acquisition systems:
ï High resolution (ground)
ï High geometric reliability
ï Relatively inexpensive
ï Day light exposure (10:00 am- 2:00 pm) required
ï Poorer contrast at shorter wavelengths
ï Film non-reusable
ï Inefficient for digital analysis