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List:       grass-user
Subject:    Re: [GRASS-user] Low pass filtering
From:       John Nicholls <johnnicholls () fastmail ! fm>
Date:       2013-01-26 12:39:44
Message-ID: 1359203984.9757.140661182624629.40512AB1 () webmail ! messagingengine ! com
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Thanks Benjamin, the stripes should be perfectly aligned along the X or
Y axis of the region for each data set, generally the data is collected
east/west or north/south. Thanks for the light reading. Will see how I
get on.

Thanks also Nikos - will dive deeper and see what comes.

JohnN.

,On Fri, Jan 25, 2013, at 09:08 PM, Nikos Alexandris wrote:
> 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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