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Remote Sensing - a tool for environmental observation

When it is stored in a digital form eg on computer

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resolution of the thermal band of Landsat TM is 120 by 120 m. When it is stored in a digital form e.g. on Computer Compatible Tape (CCT) it is resampled to a pixel size of 30 by 30 m. Figure 5.1 Arrangement of scan lines and pixels in Landsat images (Sabins, 1987). Line dropouts As was already discussed in chapter 1, Landsat MSS and Landsat TM register radiance 6 lines at a time respectively 16 lines at a time. On some image data of one image line are missing due to (temporal) failure of one of the detectors. As a result every 6th or 16th line appear black. These lines are called periodic line dropouts. The data in these scan lines is lost. Most image processing systems try to correct these lines by averaging the line above and below the lost line. Striping Striping in an image refers to a regular pattern of (horizontal) stripes in an image. Most often these stripes are caused by a slight difference in calibration of the 6 (for MSS) or 16 (for TM) sensors. A slight difference in sensitivity of the sensors might cause ‘striping patterns’ in the image. The effect can play an important role while sensing dark surfaces such as water bodies. Image enhancement e.g. by histogram stretch causes also enhancement of the striping. Striping deteriorates image classification and hampers image interpretation.
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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.
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