Remote sensing and geographic information systems
Broadly defined, remote sensing refers to any methodology employed to study the characteristics of objects from a distance. Human sight, smell, and hearing are examples of rudimentary forms of remote sensing. The type of sensing systems we treat in this chapter operate on the principle of recording electromagnetic energy over a broad range of the electromagnetic spectrum, from ultraviolet to radio wavelengths. These systems take on various forms and can be operated from aircraft or from space. Historically, the most important form of remote sensing used in forestry has been aerial photography. For example, aerial photographs have been used for decades to aid in the forest-sampling and forest mensuration processes. Aerial photographs provide an efficient means of obtaining a detailed bird’s-eye view of forest conditions on even the most extensive, remote, and treacherous of terrains. As valuable as aerial-photographic techniques have become in forest management, these procedures are limited to the study of images recorded on photographic emulsions. These materials are sensitive only to energy that is either in, or near, the visible portion of the electromagnetic energy spectrum. Sensors such as thermal scanners, multispectral scanners, and side-looking radar employ wavelengths expanded beyond the photographic range; in addition, these nonphotographic sensors typically record image data in a digital format so they can be analyzed by computer-processing techniques (I, 4, 5).