1.9 Spectral Reflectance of Land Covers

1.9 Spectral Reflectance of Land Covers

Spectral reflectance is assumed to be different with respect to the type of land cover, as explained in 1.3 and 1.8. This is the principle that in many cases allows the identification of land covers with remote sensing by observing the spectral reflectance or spectral radiance from a distance far removed from the surface.

Figure 1.9.1 shows three curves of spectral reflectance for typical land covers; vegetation, soil and water. As seen in the figure, vegetation has a very high reflectance in the near infrared region, though there are three low minima due to absorption.

Soil has rather higher values for almost all spectral regions. Water has almost no reflectance in the infrared region.

Figure 1.9.2 shows two detailed curves of leaf reflectance and water absorption. Chlorophyll, contained in a leaf, has strong absorption at 0.45 m and 0.67 m, and high reflectance at near infrared (0.7-0.9 m). This results in a small peak at 0.5-0.6 (green color band), which makes vegetation green to the human observer.

Near infrared is very useful for vegetation surveys and mapping because such a steep gradient at 0.7-0.9 m is produced only by vegetation.

Because of the water content in a leaf, there are two absorption bands at about 1.5 m and 1.9 m. This is also used for surveying vegetation vigor.

Figure 1.9.3 shows a comparison of spectral reflectance among different species of vegetation.

Figure 1.9.4 shows various patterns of spectral reflectance with respect to different rock types in the short wave infrared (1.3-3.0 m). In order to classify such rock types with different narrow bands of absorption, a multi-band sensor with a narrow wavelength interval is to be developed. Imaging spectrometers (see 2.12) have been developed for rock type classification and ocean color mapping.