Conventional Aerial Photography

Since World War I, aerial photography has evolved in two directions, larger formats for accurate mapping and cartographic purposes and smaller formats for reconnaissance usage (Warner et al., 1996). The former became standardized with large, geometrically precise cameras designed for resource mapping and military use. The science of photo-grammetry was developed for transforming airphotos into accurate cartographic measurements and maps (Wolf and Dewitt, 2000). Standard, analog aerial photography today is based on the following:

• Large-format film: panchromatic, color-visible, infrared, or color-infrared film that is 9 inches (23 cm) wide. This format is the largest film in production and common use nowadays.

• Large cameras: bulky cameras weighing 100s of kilograms with large film magazines. Film rolls contain several hundred frames. Standard lenses are 6- or 12-inch (152- or 304-mm) focal length.

• Substantial aircraft: twin-engine airplanes are utilized to carry the large camera and support equipment necessary for aerial photography. Moderate (3000 m) to high (12,000 m) altitudes are typical for airphoto missions.

• Taking photographs is usually controlled by computer programming in combination with global positioning system (GPS) to acquire nadir (vertical) shots in a predetermined grid pattern that provides complete stereoscopic coverage of the mapping area.

Large-format aerial photography is expensive—$10s to $100s of thousands to acquire airphoto coverage. This cost can be justified for major engineering projects and extensive regional mapping of the type often undertaken by provincial or national governments—soil survey, environmental monitoring, resource evaluation, property assessment and taxation, topographic mapping, and basic cartography.

An example of this approach is summarized here for production of an orthophoto atlas of the Slovak Tatra Mountains (Geodis, 2006). Original vertical photographs were taken on large-format color film from a Cessna C402B twin-engine airplane guided by GPS. The flight pattern resulted in images with 80% end and 30% side overlaps to achieve complete stereoscopic coverage of the ground area at a nominal scale of 1:23,000. All photographs were taken on a single day with optimum weather conditions—no cloud cover or ground mist.

The analog photographs were scanned and georectified to create digital orthophotos, in which lens perspective, terrain relief, and curvature of the Earth were eliminated, and the images were recast into the national coordinate system. Individual orthophotos were joined into a mosaic, which was then cut into separate map sheets for publication in the atlas at a scale of 1:15,000. Primary consumers of the orthophoto atlas are tourists, who come to the Tatra Mountains by the thousands each year to enjoy the relatively unspoiled natural environment. The first edition of this atlas was produced in 2003, when most of the lower mountain slope was forested. However, a major windstorm in November 2004 blew down forest trees over broad areas of the mountain front in the portions most frequented by tourists (Fig. 1-14). A second edition of the orthophoto atlas was produced from airphoto coverage in the summer of 2005 to document the extent of this natural event.

Analog aerial photography is mature with many cameras, films, airplanes, and other equipment readily available worldwide. It is the accepted norm by most governmental agencies and commercial enterprises. Large-format digital cameras are relatively new, and several types of optical and sensor systems are in use, such as Leica ADS-40 (linear array), DSS (one CCD array), or Z/IDMC (4 CCD arrays). These cameras operate in the same spectral range as conventional analog film, and spatial resolution and geometric fidelity are comparable. Large-format digital cameras, thus, have achieved technical parity with analog cameras. The main digital drawback at present is higher cost, but that is changing rapidly. Analog cameras will continue to be used extensively for the next few years, but gradually large-format digital cameras should come to dominate the market within the next decade.

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