Surveying ii ajith sir class2
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1. Properties used in RS for discrimination The following four properties are used for interpretation of RS information: spectral : Wavelength or frequency, refractive or emissive properties of objects during interaction of EMR spatial : Viewing angle of sensor, shape and size of the object, position, site, distribution, texture temporal : Changes in time and position which affect spectral and spatial properties polarization: Object effects in relation to the polarization conditions of the transmitter and receiver 2. Remote sensing system A typical remote sensing system consists of the following sub-systems: (a) scene (b) sensor (c) processing (ground) segment 3. The following steps indicate how remotely sensed data gets converted into useful information: 1. Source of EM energy (sun/self emission: transmitter onboard sensor). 2. Transmission of energy from the source to the surface of the earth and its interaction with the atmosphere (absorption/scattering). 3. Interaction of EMR with the earth surface (reflection, absorption, transmission) or re-emission/self emission. 4. Transmission of reflected/emitted energy from the surface to the remote sensor through the intervening atmosphere. 5. Recording of EMR at the sensor and transmission of the recorded information (sensor data output) to the ground. 6. Preprocessing, processing, analysis and interpretation of sensor data. 7. Integration of interpreted data with other data sources for deriving management alternatives and applications. 4. Interactions EMR interaction in Atmosphere Atmospheric interaction consists of the following types: Atmospheric Absorption Energy is absorbed and re-radiated again in all directions, usually over a different range of wavelengths. This is a case of radiation-matter interactions, in which the quantification of energy is important, so we will use the particle description of EMR. Atmospheric Scattering Energy is lost by redirection away from the satellite's field of view, but wavelength remains the same. 5. Interactions Irrespective of source, all radiation detected by remote sensors passes through some distance (known as the path length) of atmosphere and the net effect of the atmosphere varies with: 1.Differences in path length 2.Magnitude of the energy signal that is being sensed 3.Atmospheric conditions present 4.Wavelengths involved 6. Ray-1: Photons which leave the surface and reach sensor without change. This constitutes useful signal for remote sensing. Ray-2: Photons which leave the land/sea surface heading in the direction of the sensor but which are absorbed by interaction with the atmosphere en route. Ray-3: Photons diverted out of sensor's field of view (FOV) by scattering as a result of atmosphere interaction. 7. Ray-4: Photons of EM energy which are emitted by the atmosphere itself. Ray-5: Photons of energy from the illuminating source (sun or active radar source) which are scattered into the FOV of the sensor without touching the surface (land/sea target). Ray-6: Photons which have left the ground and carry information from an area other than the ground FOV of the sensor, and which are deflected by the atmospheric scattering into the FOV of the sensor. 8. Mechanism for absorption The primary mechanism by which the atmosphere absorbs radiation is through molecular absorption by gases. A photon, or quantum, of energy is exchanged between a molecule or atom of gas and the electromagnetic wave by following arrangements: Electron transitions: It causes promotion of electrons to higher energy orbital for absorption (lower energy orbital for emission) of EMR in the visible portion of the spectrum. Vibration of triatomic molecules: It is induced by EMR in the infrared portion of the electromagnetic spectrum. Rotation of diatomic molecules: EMR in the infrared and microwave wavelengths excites rotational motion of the molecules 9. This quantization results in an absorption spectrum for each molecule that is composed of a narrow set of absorption peaks or lines. This absorption spectrum represents wavelengths at which the corresponding energy can be absorbed. Thus for fixed E, h, and c, so one can easily identify for a given change in E 10. Atmospheric scattering Scattering in the atmosphere caused by particles such as liquid water drops, smoke, haze, and dust. Particles can absorb as well as scatter, but scattering by particles usually dominates over absorption by particles. Scattering occurs when radiation is reflected or refracted by particles in the atmosphere which may range from molecules of the constituent gases to dust particles and large water droplets. It is considered as a disturbance of the EM field by the constituents in the atmosphere resulting in the change in the direction and spectral distribution of energy in the beam. 11. Atmospheric scattering Scattered radiation, whether coming from the sun (down welling) or reflected from the earth surface (upwelling), is not attenuated but rather redirected. This redirection is wavelength-dependent. Pure scattering is said to occur in the absence of all absorption; there is no loss of energy- only redirection of energy. It must be remembered that while molecular absorption removes energy as it passes through the atmosphere and re-radiates uniformly in all directions at a different wavelength, scattering changes the direction of propagation only, not the wavelength. 12. Properties of scattering Strongly directionally dependent. Dependent on the polarization of the EMR. Dependent on the wavelength of the EMR: shorter wavelengths scatter more. Strong dependence on the size of the scattering particles relative to the wavelength of the EMR. Dependent on the density of the scattering particles: multiscatter. 13. If the particles are sparse, EMR is scattered once. The scatter primarily changes the angle of propagation, removing (attenuating) energy from the beam of radiation. 14. If the particle density is high, EMR is scattered repeatedly. This can both add and remove energy from the beam of radiation, or result in isotropic radiance. 15. Three main types of scattering: Rayleigh or Molecular scattering, when >> d Mie scattering, when ~ d Isotropic or nonselective scattering, when
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