Spectral Characteristics of Terrestrial Surfaces

Terrestrial surfaces are composed of a diverse assemblage of human-made materials such as concrete, asphalt, and metal and natural materials such as soils, shrubs, and trees. Scientists observe such features on the surface of the Earth and make measurements to better understand the characteristics associated with them. This entry describes the nature of research using remote sensing and examines the spectral characteristics of vegetation, soil, water, and human-made materials.

Characterizing and monitoring the materials through time is very important for many Earth resource management applications. While the tasks are often conducted by taking detailed field measurements, the process is time and labor intensive and is often difficult to conduct over large geographic areas. Remote sensing, which collects information about a feature without being in direct physical contact with it, has been widely used for conducting the tasks in an efficient way. Spectral remote sensor systems have matured from simple panchromatic sensors (single band in gray scale) to multispectral sensors (multiple bands, generally between 3 and 15) and further to hyperspectral or ultraspectral sensors, which collect spectral data in up to hundreds and thousands of bands in the electromagnetic spectrum. As the spectral resolution (e.g., 10 nm [nanometers]) of sensor systems improves, we can get a more accurate spectral response for an object under examination. Because each material on the surface of the Earth has a unique spectral pattern in the electromagnetic spectrum, it is important to understand the spectral characteristics of the key materials that are easily found on terrestrial surfaces. It is believed that many of the biophysical and human-made materials under investigation have unique absorption features with 10 to 20 nm width. The following sections will look into the spectral characteristics of selected natural and human-made materials, focusing on the visible, near-infrared, and middle-infrared regions (i.e., 400–2,500 nm) of the electromagnetic spectrum.