Latitude And Seasonal Conditions

The position of the sun is a critical factor for controlling the amount and quality of light available to illuminate the Earth's surface. Latitude, day of year, and time of day determine where the sun would be located for any site. The normal expectation of toplighting for small-format aerial photography means the sun should be relatively high in the sky to avoid excessive shadowing of the landscape, and a further criterion is usually cloud-free sky for best illumination of the ground. This combination of clear sky and high sun position is the ideal for most SFAP applications. In addition, it is usually desirable to avoid sun glint and hot spot views.

For some parts of the world, however, these conditions are rarely or never realized. At Troms0, Norway ( ~ 69.7°N), for example, even at noon on summer solistice, the sun is not high in the sky (Fig. 4-20). At the other extreme, the sun is nearly overhead at mid-day in the tropics, which greatly increases the chances for sun glint or the hot spot in vertical photographs. Even in middle latitudes during summer, it is possible at noon to capture the hot spot and sun glint in the same vertical view (Fig. 4-21). To avoid this predicament, SFAP should be conducted in the mid-morning or mid-afternoon, before or after the sun has reached its full height. Similar seasonal and time of day considerations apply in other circumstances in order to achieve optimum sun position for a given site or application.

The character of clouds is the second main factor for successful small-format aerial photography. Although cloud-free conditions are often assumed best, uniformly indirect lighting from high clouds or overcast sky is preferable to direct sunlight for some applications. Many parts of the world have persistent, even perennial cloud cover. This includes much of the tropical region. For example, the deltas of the Amazon River and Zaire (Congo) River are rarely photographed without cloud cover by astronauts (Amsbury et al., 1994). Tropical mountains are especially prone to cloud

FIGURE 4-20 Mid-day view of Kval0ysletta near summer solistice showing long shadows. Note poles, sign post and trees in foreground. Kite aerial photograph by JSA; near Troms0, Norway at ~69.7° N latitude, June 1998.

FIGURE 4-21 Vertical blimp aerial photograph that includes both the shadow point (left) on aquatic vegetation and sun glint (right) on still water in a wetland channel. Image acquired near noon, a few days before summer solistice at ~40°N latitude. Photograph by S. Acosta and JSA; Squaw Creek National Wildlife Refuge, northwestern Missouri, United States, June 2003.

FIGURE 4-21 Vertical blimp aerial photograph that includes both the shadow point (left) on aquatic vegetation and sun glint (right) on still water in a wetland channel. Image acquired near noon, a few days before summer solistice at ~40°N latitude. Photograph by S. Acosta and JSA; Squaw Creek National Wildlife Refuge, northwestern Missouri, United States, June 2003.

cover. Many tropical regions experience monsoon seasons with continual clouds and rain for months. Another setting known for extended cloud cover is mid- and high-latitude maritime environments, for example coastal British Columbia in Canada, southern Alaska in the United States, the British Isles, and Norway. On some islands, the sun is seen only a few days a year—Faeroe Islands (northwest of Scotland) and Kergulian Islands (southern Indian Ocean). On the other hand, deserts and semi-arid regions have abundant sunshine most of the year. However, dry climate combined with strong wind gives rise to frequent dust and sand storms (Amsbury et al., 1994).

FIGURE 4-22 Winter, leaf-off vertical image taken for property survey purposes. Picture depicts houses, garages, docks, and other lake-shore structures. Note the small cabin at scene center. During the growing season, this building is completely hidden beneath the tree canopy. Kite aerial photograph by SWA and JSA; Lake Kahola, Kansas, United States, December 2002.

FIGURE 4-22 Winter, leaf-off vertical image taken for property survey purposes. Picture depicts houses, garages, docks, and other lake-shore structures. Note the small cabin at scene center. During the growing season, this building is completely hidden beneath the tree canopy. Kite aerial photograph by SWA and JSA; Lake Kahola, Kansas, United States, December 2002.

Many regions of the world experience a regular progression of seasonal conditions that influence ground cover and human land use. Most noticeable are seasonal changes in water bodies and vegetation, both natural and agricultural. For many SFAP applications, particular criteria of ground cover may be necessary. For example, land surveys for property appraisal or population census may specify leaf-off conditions, in order that human dwellings and structures are seen clearly beneath deciduous trees (Fig. 4-22). On the other hand, agricultural monitoring must be conducted periodically during the growing season. Thus, time of year may be dictated by requirements of the SFAP mission.

As this discussion suggests, achieving optimal lighting for small-format aerial photography is often difficult or may be practically impossible in some situations. Logistical considerations provide further limitations, as people and equipment must be in place to take advantage of favorable lighting and atmospheric conditions (see Chapter 9). Every platform has its own requirements regarding weather conditions, especially wind, and the time of day for a photographic survey may be dictated by the presence or absence of wind (see Chapter 8). Given the reality of SFAP, it is sometimes necessary to proceed with less than ideal conditions in order to complete a mission. Some of the effects of cloud cover and other atmospheric disturbances are elaborated below.

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