Remote Sensing - a tool for environmental observation

The information of interest to the environmental

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The information of interest to the environmental researcher e.g. land cover, soil type, condition of the vegetation or crop or lithologic formation is deduced from these changes. Figure 1.9 shows the five most common results of radiation-matter interaction: 1. transmission, radiation is passed through the substance and very often refracted. Think of light coming from the air medium into water causing a change in velocity of the electromagnetic radiation: refraction. 2. absorption, the electromagnetic energy is absorbed by the matter and used to heat the object. 3. emitted by the substance , the electromagnetic energy is absorbed, used to heat the object and the object emits radiation according to the Stefan-Boltzmann equation. 4. scattering, the radiation is deflected in all directions. Surfaces with roughness comparable to the wavelength of the incident radiation cause scattering. 5. reflection, the incident radiation is returned from the surface with the angle of reflection equal and opposite to the angle of incidence. Reflection is caused by smooth surfaces relative to the wavelength. Diffuse reflectors or Lambertian reflectors are rough surfaces that reflect uniformly in all directions. Specular reflectors are flat surfaces that act like a mirror (angle of reflection = angle of incidence). Most earth surfaces are neither perfectly specular nor diffuse reflectors but act somewhat between these two extremes (figure 1.10). Direction of vibration or polarization may differ from the incident wave. In Remote Sensing, the differences between diffuse reflectance of objects are most often measured and interpreted. Differences of reflection by objects in the visible wavelength give the human eye the sense of colour.
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14 Do not confuse albedo and spectral reflectance. Albedo is the ratio of the total amount of electromagnetic energy reflected by a surface to the amount of energy incident upon it, most often in the spectral interval of 0.4 to 2.5 μm. Spectral reflection is the ratio of radiant energy reflected by a surface to the energy incident on it in a specific spectral interval e.g. the visible green: 0.5-0.6 μm. Reflectance is often used for both. Be always aware of the spectral region used. Figure 1.9 Interaction mechanism between electromagnetic energy and matter (Sabins, 1987). Figure 1.10 Specular and diffuse reflectors (Lillesand and Kiefer, 1994).
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15 This chapter shows that it is rather expensive and a bit cumbersome to make spectral wave- lengths visible for the human eye, which are normally not visible e.g. infrared and thermal infrared. The motivation for all these efforts is that many objects at the earth surface have a very different spectral behaviour outside the visible wavelengths. Information on the condition of vegetation and soils can much better be derived from spectral measurements in near infrared and short-wave infrared than from spectral data in the visible wavelengths. Figure 1.11 shows an example for vegetation. The differences of the spectral reflection curves of deciduous trees and coniferous trees are much larger in the near-infrared than in the visible part of the spectrum. The
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