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Banding and line drop errors detection in remotely sensed satellite data through transition count technique
Detection of Errors and Transition Count Technique:
Landsat -1,2 and 3 satellites frame covers an area of 185km. x 185km. There are 2340 scan lines, each of which has 3240 pixels contained in it. Thus, a single pixel covers about 79m x 79m on ground, which corresponds to one byte of information. In the sensing system, there are six detectors in each of four energy bands. In addition, in an individual band a strip of earth of about 480m widths is viewed in one swing of scanning mirror.
When one scene is displayed on a computer monitor, all the scan lines are displayed on the screen in a highly compact form giving an impression of a film negative captured from the space. However, because of defective data some times blank lines appears in between or some times in place of normal picture, number of bands are observed. In fact, the pixel explained above is nothing but the Grey level values of the smallest picture element in the satellite data frame. Since data is recorded in eight bits (1byte), there could be 256 possibilities of data values. These data values (Digital Number, DN) are correlated with Grey level values divided in 256 gradation levels from complete white to complete black.
Considering a single line in Landsat -1 picture frame, where 3240 pixels cover 185Km. Each pixel value is representative energy level of an area of about 79m x 79m. In view of earth surface having varied topography, the energy levels received from different parts are in different quantum and hence the pixel values along a scan line are bound to be different except on uniform surface, which is an imaginary condition. Therefore, a line containing similar pixel values does not exist in nature; this is an indicator of the error. Since satellite data is recorded in DN values thus, by comparing these values one can find information if there is any such blank line (line drop) or shaded line (banding). Technique of TC, coupled with EXOR-ing (if required) each pixel of a scan line of satellite scene with the initial pixel can provide such information. The line drop will exist if there is TC = zero, the DN number exists at the extreme of the considered pixel levels. However, in the case of banding error the TC may have any value in between 0 to 7. Which will need that the DN number pertaining to each pixel of a scan line (3240) is compared with the DN value of first pixel. If uniform DN numbers are obtained, that could be a banding error. |
