| Re: MEDIAN filtering and why it's not working [message #12061 is a reply to message #12046] |
Thu, 11 June 1998 00:00   |
thompson
Messages: 584
Registered: August 1991
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Senior Member |
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"T Bowers" <tbowers@nrlssc.navy.mil> writes:
> Hi,
> How come
> print, median([0.0,1.0,1.5,2.0,5.0,6.0], 3, /EVEN)
> prints
> 0.00000 1.00000 1.50000 2.00000 5.00000 6.00000
> instead of
> 1.00000 1.00000 1.00000 5.00000 5.00000 5.00000
> Doesn't look like it's filtering anything to me, no matter what
> I set the filter width to.
> Actually, I'd really like it to print
> 1.00000 5.00000
> just the median of each filter width. I'm trying to filter spikes in
> time series data.
> I'm using IDL 5.02 and win95
One of the oddities of the median filter program is that it doesn't filter
points near the edges of the array. It doesn't filter points if there aren't
enough points on either side. In your example, with a filter width of three,
then it won't filter the points on either end, because it needs one point
before and one point after for each point it filters.
If I understand you correctly, I think you want to do something like the
following. It would require that array be evenly divisible by the filter
width.
array = [0.0,1.0,1.5,2.0,5.0,6.0]
n = n_elements(array) / 3
array = reform(array, 3, n, /overwrite)
for i=0,n-1 do array(*,i) = median(array(*,i))
array = reform(array, 3*n, /overwrite)
This would give you
1.00000 1.00000 1.00000 5.00000 5.00000 5.00000
as you request. However, the last step would probably be better implemented as
array = reform(array(0,*), /overwrite)
to remove the repeated entries. This would give you simply
1.00000 5.00000
William Thompson
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