I've noticed a 'dropped line' in only one band of SPOT MS image, as seen similar to added example. By applying fourier transformation to the band in SAGA GIS, the dropped line can be clearly spotted trending in a vertical direction. enter image description here

Could it be eliminated making use of fourier spectrum in SAGA or QGIS?

an example of dropped line


I was able to get rid of dropped line making use of Remove Outliers available in ImageJ. Here are the results; Comparison

The data I am using can be accessed from "Flood hazard analysis using multi-temporal SPOT-XS imagery" application guide. The dropped line appears in 'DRY87_1W (Green band).

1 Answer 1


It is worth noting that the strip (dropped line) you are observing is likely an error created during the collection process and is not likely to have happened down stream. It is worth checking though.

Rather than using the Fourier spectrum you might also want to consider just "inpainting" the missing data with data from the surrounding cells but appreciate this doesn't always work depending on the resolution and data integrity.

To perform the Fourier technique you should be able to do the following using the Grass suite of tools:

  1. Inspect the Fourier spectrum to identify the frequency at which the dropped line appears.
  2. Use the "Raster Calculator" tool in the "Raster" menu to create a new raster layer that filters out the frequency of the dropped line. You can use something like "band@1 - <frequency>" where frequency is the frequency of the dropped line in the Fourier spectrum.
  3. Run the "Inverse Fast Fourier Transform (IFFT)" tool in the "GRASS" menu on the filtered raster layer to create a new raster layer with the dropped line removed.
  4. Use the "Merge" tool in the "Raster" menu to merge the original raster layer with the dropped line removed layer.
  5. Use the "Clip Raster by Extent" tool in the "Raster" menu to remove the dropped line.
  • I have edited my question and shared the link destined to faulty image I am dealing with. The noise appears as narrow vertical strip of pixels in Fourier spectrum, there is no 'single frequency value' to subtract from transformed image and then run IFFT. Is there any way to apply some denoise filter directly on Fourier spectrum to suppress these frequencies?
    – Rex
    Jan 24, 2023 at 7:58
  • QGIS doesn't have the best suite of tools to apply filtering. You could try using the GRASS r.neighbors and applying a Median filter or use SAGA Gaussian Filter or Majority/Minority Filter
    – AWGIS
    Jan 24, 2023 at 12:31
  • I'll re-try making use of suggested filters directly on Fourier Spectrum. I think I've to create a mask delimiting immediate neighbourhood of the dropped line before filtering in SAGA. You might have noticed that data supplied with the exercise is devoid of spatial references making it hard to handle in GRASS GIS.
    – Rex
    Jan 24, 2023 at 15:27

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