[GRASS-user] Low pass filtering

[Nikos Alexandris]

John Nicholls wrote:

> > Hi
> > I am looking for some direction in applying a low pass filter to a
> > raster greyscale of geophysics data. The filtering objective is fairly
> > typical, aimed at removing influence of background noise normally

Benjamin Ducke wrote:

> Striping is not really "noise". It's a systematic defect
> in your data that cannot easily be removed with a simple
> low-pass filter.
> 
> There is really no universal cure for this problem.
> Unless your stripes are perfectly aligned with the
> X or Y axis of your geographical region, simple
> map algebra won't get you far, either.
> 
> The classic treatise on the subject is this one:
> http://www.oimoen.com/PDFs/artifacts.pdf
> 
> Basically, the "trick" is using a low-pass and high-pass
> filter whose shapes and sizes match those of your
> stripes.

Depending on the data to be processed, may I add that it might be useful to 
try PCA (i.pca, noise in general and, perhaps, striping effects, might appear 
in some of the higher order components which can be rejected) and or FFT 
(i.fft, apply low-high pass filters in the frequency domain?, then jump back to 
the spatial domain).

Best, Nikos

[..rest deleted..]
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