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Remote Sensing - a tool for environmental observation

Image enhancement e.g by histogram stretch causes

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Unformatted text preview: Image enhancement e.g. by histogram stretch causes also enhancement of the striping. Striping deteriorates image classification and hampers image interpretation. 64 Figure 5.2 Digital structure of a Landsat TM image. 5.3 Radiometric Corrections Radiometric correction is an image restoration operation. Image restoration techniques aim at compensating for errors, noise and distortion by scanning, transmission and recording images. Sources of radiometric distortion are the atmosphere and the instruments used to register the image data (Richards, 1986). Figure 5.3 shows the effect of the atmosphere on the measured brightness of one single pixel. The sun radiates directly electromagnetic to the pixel at a certain location but this pixel also receives scattered radiance from the sky (sky irradiance 1) and scattered radiance from neighbouring pixels (sky irradiance 2). The amount of scattering varies strongly in space and time and depends on the condition of the atmosphere (see also section on Rayleigh and Mie scatter). Remember that Rayleigh and Mie scatter are wavelength dependent and that their effects will be different in the different wavebands of a remote sensing system. The sensor measures radiance directly originated from the pixel (figure 5.3), but it also receives radiance scattered in the atmosphere (path radiance 1) and radiance reflected from neighbouring pixels (path radiance 2). Radiometric errors can also be caused by the design and operation of the sensor system. The most significant of these errors is related to the detector system. An ideal radiation detector should have a linear transfer characteristic (i.e. radiation in --> signal out) as shown in figure 5.4. However, real detectors have some degree of non-linearity and will give a small signal out even when no radiation is detected: dark current, or the offset in figure 5.4. The slope of the characteristic is called transfer gain or gain. Consequently, two steps are generally necessary for radiometric correction 1) correction for gain and offset of the detector and 2) correction for the atmospheric conditions at the time of data acquisition. Two approaches are possible for radiometric corrections: 65 1) if sufficient measurements of the conditions of the atmosphere such as optical thickness, scattering and sun elevation are available, and if the surface reflectance of several dark and bright targets in the study area are determined, then the image can be radiometrically corrected using models such as the 5S code, LOWTRAN or ATCOR. A detailed description of the atmospheric effect on remote sensing imagery is provided by Kaufman (1989), Kaufman and Sendra (1988), Richards (1986) and Lillesand and Kiefer (1994, p.531-536). Model descriptions of LOWTRAN and 5S-code are available from Kneizys et al. (1988) and Tanré et al. (1985)....
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