UNIT 4 Electromagnetic Spectrum

UNIT 4 Electromagnetic Spectrum - 1 GIS 4037c Remote...

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GIS 4037c Remote Sensing of the Environment Dr. Charles Roberts THE SPECTRUM, RADIATION IN THE ATMOSPHERE, AND RADIATION AT THE SURFACE The Electromagnetic Spectrum For all practical purposes, remote sensing is the sensing of electromagnetic wavelengths of energy. Remotely sensed imagery are essentially maps of Electromagnetic Radiation (EMR) as it interacts with the earths surface. When we examine satellite imagery or aerial photographs, we are essentially looking at either the reflectance of EMR from the surface of the earth (Aerial Photographs) or from the cloud tops (GOES weather satellite) or else we are examining the emittance of EMR from the surface or cloud tops of the earth (Thermal imagery). The important point is that there is a distinction between reflected radiation and emitted radiation is critical. Our eyes and brain are a mechanism for the remote sensing of a small portion of the electromagnetic spectrum. We call this portion the "visible" part of the spectrum. Other creatures can see and feel other portions of the spectrum: -Pit Vipers: Thermal RS -Lesser Nighthawks and Bats: active rs, essentially sonar -Whales and Dolphins, active remote sensing, Sonar -Deer: Ultraviolet Electromagnetic energy is generated by several processes: -A change in the energy levels of electrons -Acceleration of electrical charges -Decay of radioactive substances -Thermal motion of molecules The sun (a nuclear reactor) produces a full spectrum of EMR. When this radiation passes through our atmosphere, one of three things can happen to it: -it is reflected and scattered from cloud tops and the atmosphere -it is transmitted to the earths surface where it is either -reflected, transmitted or absorbed. If it is absorbed, it must be emitted Notice that we can either sense 1) Reflected radiation or 2) Emitted radiation. To understand remotely sensed imagery, it is necessary to consider the nature of EMR 1
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here. Later we will return to examine particular characteristics of electromagnetic wavelengths. EMR occurs over a wide range of frequencies from long (hundreds of kilometers) low frequency radio waves to very short high frequency gamma and cosmic rays. In common practice, this electromagnetic spectrum is divided into various wavelengths bands or spectral regions, such as X-rays, ultraviolet rays, blue rays, green rays, red rays, infrared rays and microwaves, etc. In reality, none of these regions have distinct or discreet boundaries, and the limits of both short and long wavelengths are not known precisely. Most remote sensing work involves the use of visible, near (reflected infrared), far infrared, and microwave. Here we can make a distinction between reflected and emitted radiation: -Short wave: reflected -Long wave: emitted The boundary between the two is in the infrared. There is also a distinction between: -Near Infrared: (reflected) -Very Near Infrared -Short Wave Infrared -Far or Thermal Infrared: (emitted) -Microwaves are close to thermal infrared.
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