Actual SFAP

• Sunlight: varies with time and place in ways that cannot be fully predicted. Calibration is sometimes possible, but the exact nature of available solar energy is usually not known.

• Atmosphere: varies according to latitude, season, time of day, local weather, etc. Selective absorption and scattering are the rule at most times and places.

• Spectral signatures: all objects have theoretically unique signatures, but in practice these may change and cannot always be distinguished; many objects appear the same.

• Real cameras: no existing small-format camera system can operate practically in all wavelengths of interest. Each camera is limited by its optical, film, or electronic nature to certain wavelengths. Likewise, certain cameras are limited by their high cost.

• Data handling: many cameras now generate far more imagery than can be handled instantly by either visual inspection or computer analysis.

• Multiple users: no single combination of imagery and analysis satisfies all users. Many users are not familiar with subjects outside their immediate disciplines and thus cannot appreciate the full potential or limitations of small-format aerial photography.

Small-format aerial photography, like other types of remote sensing, is a compromise between the ideal and what is logistically feasible and financially affordable for a given project. In this regard, the relatively low cost, high spatial resolution, and field portability of SFAP offer some advantages not possible with other means of aerial remote sensing.

SFAP normally exploits the so-called visible atmospheric window consisting of wavelengths from ~0.3 to 1.5 mm long (Fig. 2-1). On a cloud-free day, this range of wavelengths passes through the atmosphere with little scattering or absorption by gas molecules, aerosols, or fine

Small-Format Aerial Photography

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lens relative to the horizon (Fig. 2-2). The amount of tilt is called depression angle.

• High-oblique vantage: side view, horizon is visible, depression angle typically <20°.

• Low-oblique vantage: side view, horizon is not visible, depression angle typically 20°-87°.

• Vertical vantage: view straight down, also called nadir, depression angle >87°-90°.

Vertical images are generally preferred for mapping and measurement purposes, as explained below, because the geometry of vertical images can be calculated. However, such vertical views may be difficult for many people to interpret unless they are quite familiar with the site and objects shown in the image. Oblique shots, on the other hand, provide overviews of sites and their surroundings that are quite easy for most people to recognize visually and understand readily (e.g. Ham and Curtis, 1960). Yet, oblique photographs have substantial distortions in scene geometry that render accurate measurements difficult or impossible.

FIGURE 2-1 Spectrum of visible light in micrometers (mm) wavelength. All visible colors are made up of three primary colors: blue (0.4-0.5 mm), green (0.5-0.6 mm), and red (0.6-0.7 mm). Near-ultraviolet is ~0.3 to 0.4 mm, and near-infrared is ~0.7 to 1.5 mm wavelengths.

dust. Given the low-height operation for most SFAP below 300 m or even <100 m, images are acquired in which the reflected radiation has suffered minimal degradation from atmospheric scattering or absorption. This is an important consideration in terms of spectral signatures of objects depicted in SFAP images.

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