We've sometimes seen unexpected results from low pass filtering on line
data. Sometimes we've employed
polynomial fitting instead or if possible worked on a gridded version of the
data. MAGMAP filters seem to
work very predictably.
Best Regards,
Chris Vaughan
President and Chief Geophysicist
CGI Controlled Geophysics Inc.
Thornhill, Ontario, CANADA
[EMAIL PROTECTED]
----- Original Message -----
From: Marc Pelletier <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Thursday, June 29, 2000 1:07 PM
Subject: [geonet]: Stupid filter question
> Hello all,
>
> I am doing some tie line levelling. In order to create a smooth correction
function to apply to my data I use a combination of a binary spline and a
low pass filter on my intersection errors. My question is about the low pass
filter. It seems that there is a very definite limit on the useful length of
it. I've uploaded an example to http://www.datadonkey.com/filter.htm. In
this case the line length is about 3000 fids and the green dots
(intersection errors) are about 50 fids apart. The purple line is the result
of the binary spline (settings of .76 and .9) The last grey line, which you
can barely see, is the filtered result of the purple line. In this example
the low pass cutoff is set at 500. I expected the data to get pretty much
whacked at that length. I've tried varying the length from 50 to 5000(!) and
the results are always the same. The difference is in the second decimal
place.
>
> Now, I'm not looking for filtering or levelling advice here. It just seems
to me that the cutoffs don't work the way I would expect on low pass
filtering. Can anyone comment?
>
> Thanks
>
> Marc Pelletier
>
> --------------------------------------------------------------------------
-
> Data Donkey - Geophysical Data Processing, Consulting and Software
Development
> Creator of the POWER TOOLBOX utilities for OASIS montaj
> contact: mailto:[EMAIL PROTECTED] tel (306) 931-6853 or
http://www.datadonkey.com
>
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