Surveying ii ajith sir class1

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GCE Kannur

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  • 1. REMOTE SENSING

2. Introduction Remote sensing is a general term which describes the action of obtaining information about an object with a sensor which is physically separated from the object. Such sensors rely upon the detection of energy emitted from or reflected by the object. Two common examples of remote sensing are human vision, which relies on the detection of reflected light, and sonar, which detects sound waves. 3. A formal and comprehensive definition of applied remote sensing Remote Sensing in the most generally accepted meaning refers to instrument-based techniques employed in the acquisition and measurement of spatially organized (most commonly, geographically distributed) data/information on some property(ies) (spectral; spatial; physical) of an array of target points (pixels) within the sensed scene that correspond to features, objects, and materials, doing this by applying one or more recording devices not in physical, intimate contact with the item(s) under surveillance (thus at a finite distance from the observed target, in which the spatial arrangement is preserved); techniques involve amassing knowledge pertinent to the sensed scene (target) by utilizing electromagnetic radiation, force fields, or acoustic energy sensed by recording cameras, radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, sound detectors, seismographs, magnetometers, gravimeters, 4. Basic Components All remote sensing technologies are based on certain common concepts, and all remote sensing systems consist of the same basic components. These four basic components of a remote sensing system include a target, an energy source, a transmission path, and a sensor. 5. Basic Components The target is the object or material that is being studied. The components in the system work together to measure and record information about the target without actually coming into physical contact with it. There must also be an energy source which illuminates or provides electromagnetic energy to the target. 6. Target Interactions The energy interacts with the target, depending on the properties of the target and the radiation, and will act as a medium for transmitting information from the target to the sensor. The sensor is a a remote device that will collect and record the electromagnetic radiation. Sensors can be used to measure energy that is given off (or emitted) by the target, reflected off of the target, or transmitted through the target. 7. Target Interactions Once the energy has been recorded, the resulting set of data must be transmitted to a receiving station where the data are processed into a usable format, which is most often as an image. The image is then interpreted in order to extract information about the target. This interpretation can be done visually or electronically with the aid of computers and image processing software. 8. Classification 1. In respect to the type of Energy Resources Active remote sensing Passive remote sensing 2.In respect to Wavelength Regions: Remote Sensing is classified into three types in respect to the wavelength regions Visible and Reflective Infrared Remote Sensing. Thermal Infrared Remote Sensing. Microwave Remote Sensing. 9. Active remote sensing Direct radiation of a particular form towards an object and then detect the amount of that energy which is radiated by the object. These active remote sensing systems operate in the microwave and radio wave regions of the EM spectrum. Lidar (laser imaging radar) systems are active remote sensors which operate in the ultraviolet, visible and near infrared wavelengths. 10. Passive remote sensing Relies on the radiation originating from some other source, principally the sun. Reflected solar energy is detected by passive remote sensing devices in the visible, near infrared and middle infrared regions while the Earth's emitted energy may be detected in the middle infrared and thermal infrared wavelengths. Certain microwave sensors are also in the passive detector category. Aerial photography and Landsat satellite imagery are examples of data collected by passive remote sensing systems. 11. Electromagnetic Energy The underlying basis for most remote sensing methods and systems is simply that of measuring the varying energy levels of a single entity, the fundamental unit in the electromagnetic (which may be abbreviated "EM") force field known as the photon 12. Electromagnetic Spectrum Variations in photon energies (expressed in joules or ergs) are tied to the parameter wavelength or its inverse, frequency. EM radiation that varies from high to low energy levels comprises the electromagnetic spectrum (EMS). Radiation from specific parts of the EM spectrum contain photons of different wavelengths whose energy levels fall within a discrete range of values. 13. Electromagnetic energy and interactions Radiation from specific parts of the EM spectrum contain photons of different wavelengths whose energy levels fall within a discrete range of values. When any target material is excited by internal processes or by interaction with incoming EM radiation, it will emit or reflect photons of varying wavelengths whose radiometric quantities differ at different wavelengths in a way diagnostic of the material. 14. Energy Interactions, Spectral Reflectance and Colour Readability in Satellite Imagery All matter is composed of atoms and molecules with particular compositions. Therefore, matter will emit or absorb electro- magnetic radiation on a particular wavelength with respect to the inner state. All matter reflects, absorbs, penetrates and emits Electro-magnetic radiation in a unique way. Electro- magnetic radiation through the atmosphere to and from matters on the earth's surface are reflected, scattered, diffracted, refracted, absorbed, transmitted and dispersed. For example, the reason why a leaf looks green is that the chlorophyll absorbs blue and red spectra and reflects the green. The unique characteristics of matter are called spectral characteristics. 15. Any beam of photons from some source passing through medium 1 (usually air) that impinges upon an object or target (medium 2) will experience one or more reactions 16. Electromagnetic Energy The sun provides most of the energy which we sense as light. This energy consists of electromagnetic (EM) waves which travel in harmonic, sinusoidal motion as shown. 17. Electromagnetic Spectrum "Electromagnetic radiation is energy propagated through space between electric and magnetic fields. The electromagnetic spectrum is the extent of that energy ranging from cosmic rays, gamma rays, X-rays to ultraviolet, visible, and infrared radiation including microwave energy." Electromagnetic Waves Electromagnetic waves may be classified by frequency or wavelength, and the velocity of ALL electromagnetic waves is equal to the speed of light, which we (along with Einstein) will refer to as c. 18. Electromagnetic energy is continuously emitted at all wavelengths by every material with a temperature above absolute zero (-273.15C or 0 K). With no other objects in the universe, a material would gradually cool to 0 K by radiating all of its energy. Absorption of energy increases both the temperature and rate of emission of a material. If the material is 'black' in that it absorbs all radiation that reaches it (a perfect absorber is referred to as a 'blackbody'), then the spectral composition and intensity of emission are well defined and follow Planck's laws 19. Because the Earth, and its surface materials, are not black, they do not absorb all the sun's radiation but reflect and scatter radiation as well. The spectral composition of radiation from the Earth therefore consists of both reflected and emitted components. The intensity of radiation emitted from Earth, when viewed from space, is greatest where the sun's radiation is greatest (that is, in the visible region) due to the reflectance of solar energy. 20. Much of the longer wavelength energy cannot be seen or photographed but can be sensed with radiometers and scanners. The range of wavelengths in which various sensors can operate is shown in Figure 21. The interaction of incoming radiation with surface features depends on both the spectral reflectance properties of the surface materials and the surface smoothness relative to the radiation wavelength. A relatively 'smooth' surface which reflects energy without any scattering (that is, the angle of incidence equals the angle of reflection) is called a 'specular' or 'mirror' reflector. 'Diffuse' or 'Lambertian' reflectance occurs when the surface is rough relative to the wavelength(s) of the incoming radiation and causes the energy to be reflected equally in all directions.