[GRASS-user] Low pass filtering

[Benjamin Ducke]

On 01/25/2013 05:28 PM, 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

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.

> visible as linear striping or similar in a raster image created via
> v.in.ascii then v.surf.idw or v.surf.bspline. I’ve tried r.mfilter
> applying both 3x3 and 7x7 low pass filters as detailed in the GRASS
> literature, and ask here if there are other methods either via r.mapcalc
> or R which may prove more effective. I am not familiar with r.mapcalc,
> nor R. I  wonder if anybody has applied lowpass filters using either of
> these 2 before, and would be kind enough to provide me with a lead in to
> get started. Perhaps there a manual page I've not yet got to. Still a
> long way to go with GRASS, but it  really is superb for the limited
> vector and raster analysis I need.

It's also superb for very advanced vector and raster
analyses ;)

Cheers,

Ben

>
> Grass version I am using is 6.4.2 on Ubuntu.
>
> Many thanks in advance,
>
> JohnN
>



-- 
Benjamin Ducke
{*} Geospatial Consultant
{*} GIS Developer

   benducke at fastmail.fm