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with a high transmission could be potential candidates for remote sensing missions.

1.1.2 Surface material reflectance

The choice of wavebands for remote sensing in the optical region is also affected by the characteristics of the surface material. When energy is incident upon a target, that energy can be separated into three portions, namely, energy that is transmitted, absorbed, and reflected. Surface material reflectance characteristics may be quantified by the spectral reflectance, which is a percentage measure obtained simply by dividing reflected energy in a given waveband by incident energy. The quantity of the reflected energy depends mainly on three factors: the magnitude of the incident energy, the roughness of the material, and the material type. Normally, the first two factors are regarded as constants. Therefore, only the third factor, i.e. the material type, is considered. However, it is worthwhile to describe first how roughness affects the reflected energy.

Surface roughness is a wavelength-dependent phenomenon. Given the same material, the longer the wavelength the smoother the material appears. For a perfectly smooth (specular) surface, reflected energy travels only in one direction such that the reflection angle is the same as incidence angle. In the case of a perfectly rough (Lambertian) surface, incident energy is reflected equally in all directions (Figure 1.4). However, in practical applications, most surface materials act neither as specular nor Lambertian reflectors; their roughness lies somewhere between these two extremes.

Figure 1.5 shows the average reflectance over the optical region of the spectrum for three ideal surface materials: dry bare soil, clear water and green vegetation. This graph shows how these surface materials can be

Figure 1.4 (a) Specular reflectance; and (b) Lambertian reflectance.

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Classification Methods for Remotely Sensed Data
Classification Methods for Remotely Sensed Data, Second Edition
ISBN: 1420090720
EAN: 2147483647
Year: 2001
Pages: 354

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