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Rs and gis lect 3-6.pdf

  1. Electromagnetic spectrum, Principal applications of different wavelength regions Dr. Sanjani S. Salunkhe, Assistant Professor, Dept. of Soil and Water Cons. Engg., Dr. D.Y.Patil CAET, Talsande Lec. 3-6
  2. Electromagnetic Remote Sensing Process • The generalised processes involved in electromagnetic remote sensing system or passive remote sensing system, namely, data acquisition and data analysis.
  3. Fig . 1: Electromagnetic Remote Sensing Process with overview on GIS
  4. The data acquisition process comprises distinct elements, namely, a) energy sources, b) propagation of energy through the atmosphere, c) Energy interactions with earth's surface features d) airborne/space borne sensors to record the reflected energy and e) generation of sensor data in the form of pictures or digital information.
  5. Energy Sources and Radiation Principles • Visible light is only one of many forms of electromagnetic energy. • Radio waves, ultraviolet rays, radiant heat, and X-rays are other familiar forms. • All this energy is inherently similar and propagates in accordance with basic wave theory. • This theory describes electromagnetic energy as traveling in a harmonic, sinusoidal fashion at the “velocity of light” c.
  6. Wave theory • Radiation is characterized with respect to frequency and wavelength. • Waves obey the general equation, C = νλ where, C is velocity of light 3.0 x 108 m/s, ν is frequency and λ is wavelength Fig. Electromagnetic wave
  7. Electromagnetic spectrum • Electromagnetic spectrum continuation of the energy that ranges from meters to nanometers in wavelength, travels, at the speed of light and propagates thorough a vacuum such as outer space. Fig.3: Electromagnetic spectrum
  8. Region Wavelength Remarks Gamma ray <0.03 nm Incoming radiation is completely absorbed by upper atmosphere and is not available for remote sensing X- ray 0.03 to 3.0 nm Completely absorbed by atmosphere. Not employed in remote sensing Ultraviolet 0.03 to 0.4 µm Incoming wavelength less than 0.3µm are completely absorbed by ozone in the upper atmosphere Photographic UV band 0.3 to 0.4 µm Transmitted through atmosphere. Detectable with film and photo-detector, but atmospheric scattering is severe. Visible 0.4 to 0.7 µm 0.4- 0.5 µm (Blue) 0.5- 0.6 µm (Green) 0.6- 0.7 µm (Red) Imaged with film and photo-detector. Includes reflected energy peak of earth at 0.5 µm. Reflected IR band 0.7 to 3.0 µm 0.7 – 1.3 µm (NIR) 1.3- 3.0 µm (MIR) The band from 0.7 to 0.9 µm is detectable with film and is called photographic IR band. Thermal IR band 3 to 5 µm 8 to 14 µm Principal atmospheric window in thermal region. Images at these wavelengths are acquired by optical mechanical scanners and special vidicon systems but not by film. Microwave/ Radar 1 mm to 1 m Longer wavelengths can penetrate clouds, fog, and rain. Images may be acquired in the active and passive mode. Electromagnetic spectral regions Book no. 4, Table 1.3
  9. Particle theory • This theory suggests that electromagnetic radiation is composed of many discrete units called photons or quanta. The energy of quantum is given by, Q = hv Where Q is energy of a quantum, Joules, h is Plank’s constant, 6.626 x 10-34 J sec, v is frequency. • The wave and quantum models of EM radiation can be related as, Q = hc/ λ
  10. Stefan-Boltzmann Law • All matter at temperatures above absolute zero (-273o C) continually emit EM radiation. • Like the sun, terrestrial objects are also sources of radiation, though of a different magnitude and spectral composition than that of the sun. The amount of energy that any object radiates can be expressed by: M = σT 4 M = total radiant exitance from the surface of a material (W m-2) σ= Stefan-Boltzmann constant, (5.6697 x 10-8 W m -2 K -4 ) T = absolute temperature (K) of the emitting material
  11. Weins Displacement law • The spectral distribution of energy varies also with temperature. • The dominant wavelength or wavelength at which a blackbody radiation curve reached a maximum, is related to temperature by Weins displacement Law: λm = A /T λm = wavelength of maximum spectral radiant exitance,μm A = 2898 mm, K T = Temperature, K
  12. Black Body Radiation as a function of Temperature and Wavelength Fig.4: Spectral distribution of energy radiated from blackbodies of various temperatures
  13. • The sun emits radiation in the same manner as a blackbody radiator whose temperature is about 6000 K (Fig.4). • Many incandescent lamps emit radiation typified by a 3000 K blackbody radiation curve. • The earth’s emits radiation is about 300 K (Fig.4).
  14. Energy Interactions in the Atmosphere • All radiation detected by remote sensors passes through some distance, or path length, of atmosphere. The path length involved can vary widely. • For example, space photography results from sunlight that passes through the full thickness of the earth’s atmosphere twice on its journey from source to sensor. • An airborne thermal sensor detects energy emitted directly from objects on the earth, so a single, relatively short atmospheric path length is involved.
  15. • Atmospheric scattering is the unpredictable diffusion of radiation by particles in the atmosphere. • Three types of scattering can be distinguished, depending on the relationship between the diameter of the scattering particle (a) and the wavelength of the radiation (λ). Scattering
  16. • Rayleigh scatter is common when radiation interacts with atmospheric molecules (gas molecules) and other tiny particles (aerosols) that are much smaller in diameter that the wavelength of the interacting radiation. • The effect of Rayleigh scatter is inversely proportional to the fourth power of the wavelength. As a result, short wavelengths are more likely to be scattered than long wavelengths. • Rayleigh scattering is the dominant scattering mechanism in the upper atmosphere. • Rayleigh scatter is one of the principal causes of haze in imagery. Rayleigh Scattering a < λ
  17. • Mie scattering occurs when the particles are just about the same size as the wavelength of the radiation. • Dust, pollen, smoke and water vapour are common causes of Mie scattering which tends to affect longer wavelengths than those affected by Rayleigh scattering. • Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud conditions are overcast. Mie Scattering a < = > λ
  18. • Non-selective scattering occurs when the particles (e.g. water droplets and large dust particles ) are much larger than the wavelength of the radiation. • Non-selective scattering gets its name from the fact that all wavelengths are scattered about equally. • This type of scattering causes fog and clouds to appear white to our eyes because blue, green, and red light are all scattered in approximately equal quantities. Non-selective scattering a > λ
  19. • In contrast to scatter, atmospheric absorption results in effective loss of energy to atmospheric constituents. Generally involves absorption of energy at a given wavelength. • The wavelength ranges in which the atmosphere is particularly transmissive of energy are referred as atmospheric windows. Absorption
  20. • When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible (an element of the volume of a water body) • Various fractions of the energy incident on the element are reflected, absorbed, and/or transmitted. Energy interactions with earth surface features Fig. 5: Basic interactions between electromagnetic energy and an earth surface feature
  21. • Fractions of the energy are reflected, absorbed, and/or transmitted. • Applying the principal of conservation of energy, we can state the interrelationship between these three- energy interactions as Where, EI denotes the incident energy, ER denotes the reflected energy, EA denotes the absorbed energy and ET denotes the transmitted energy, with all energy components being a function of wavelength.
  22. • The reflectance characteristics of earth surface features may be quantified by measuring the portion of incident energy that is reflected. • This is measured as a function of wavelength and is called spectral reflectance, • A graph of the spectral reflectance of an object as a function of wavelength is termed a spectral reflectance curve. Spectral reflectance
  23. • Fig. assume that task of selecting an airborne sensor system to assist in preparing a map of a forested area differentiating deciduous versus coniferous trees. Fig. Generalized spectral reflectance envelopes for deciduous (broadleaved) and coniferous (needle-bearing) trees
  24. Electromagnetic energy • It refers all energy that moves with the velocity of light in a harmonic wave pattern. • Each frequency is associated with a different standing wave pattern. These frequencies and their associated wave patterns are referred to as harmonics.
  25. • In interaction, electromagnetic energy behaves as through it consists of many individual bodies called as photons that have such particle like properties as energy and momentum. • When light refracts as it moves through media of different optical densities, it is behaving like waves.
  26. 1. Transmitted, that is passed through substance. The ratio of two velocities called index of refraction (n), n = Ca/Cs Ca = velocity in a vacuum, Cs = velocity in substance 2. Absorbed, giving up its energy largely to matter. 3. Emitted by substance, usually at longer paths a function of its structure and temperature. 4. Scattered, that is, deflected in all directions with dimensions of relief, or roughness, comparable to the wave length of incident energy produce scattering. 5. Reflected, that is, returned form surface of material with angle of reflection equal and opposite to angle of incidence.
  27. References 1. Book no. 3: 1.1-1.4 2. Book no. 4: chap. 1
  28. Questions 1. Explain the electromagnetic Remote Sensing Process. 2. Explain wave theory and particle theory. 3. Give Stefan-Boltzmann Law and Weins Displacement law. 4. Define scattering and explain its types.
  29. Thank You