Correction of periodic line dropouts

Periodic line dropouts occur due to recording problems when one of the detectors of the sensor in question either gives wrong data or stops functioning. The Landsat-7 ETM, for example, has 16 detectors for each of its channels, except the thermal channel. A loss of one of the detectors would result in every sixteenth scan line being a string of zeros that would plot as a black line on the image (see Figure 1).

The first step in the restoration process is to calculate the average DN value per scan line for the entire scene. The average DN value for each scan line is then compared with this scene average. Any scan line deviating from the average by more than a designated threshold value is identified as defective. In regions of very diverse land cover, better results can be achieved by using the histogram for sub-scenes and processing these sub-scenes separately.

The next step is to replace the defective lines. For each pixel in a defective line, an average DN is calculated from the DNs for the corresponding pixel in the preceding and succeeding scan lines by using the principle of spatial autocorrelation. The average DN is then substituted for the defective pixel. The resulting image is a major improvement, although every sixteenth scan line (or every sixth scan line, in the case of Landsat MSS data) consists of artificial data (Figure 2). This restoration program is equally effective for random line dropouts that do not follow a systematic pattern.