# [ncl-talk] Filter, filwgts_lanczos

yang yang yang.yang816 at gmail.com
Thu Oct 29 16:18:18 MDT 2015

```Hi Maria:

Thanks very much for the information and your help!

Ed

On Fri, Oct 30, 2015 at 10:03 AM, Maria Gehne - NOAA Affiliate <
maria.gehne at noaa.gov> wrote:

> Hi Ed,
>
> if you look at the filter weights example on the NCL website:
> http://www.ncl.ucar.edu/Document/Functions/Built-in/filwgts_lanczos.shtml
>
> you can see an example of how to compute the time fca value. You can do
> exactly the same for space.
>
> time:
>
> dt = 3 days per time step
>             t1 = 50 days  (low frequency cutoff, expressed in time domain)
>             t2 = 10 days  (high frequency cutoff, expressed in time domain)
>
>             fca = dt/t1 = 3./50. = 0.06 time steps  (low frequency cutoff)
>             fcb = dt/t2 = 3./10. = 0.30 time steps  (high frequency cutoff)
>
>
> space:
>
> dx = 100m,  x1= 500m (high frequency cutoff), fca = dx/x1 = 100/500= 1/5 = 0.2
>
>
> Hope this helps,
>
>
> Maria
>
>
>
> On Tue, Oct 27, 2015 at 3:42 PM, yang yang <yang.yang816 at gmail.com> wrote:
>
>> Hi Dennis:
>>
>>
>> *In *filwgts_lanczos  a parameter, *fca* should be given. That
>> is a scalar indicating the cut-off frequency of the ideal high or low-pass
>> filter: (0.0 < *fca* < 0.5). In a 1-D spatial data, I want to filter
>> out small scale or wavelength waves. For my case, I think that fca should
>> be wavelength or wave numbers, instead of frequency. Suppose I have 300
>> data with regular 100 m spatial interval. I would to remove
>> wavelength smaller than 500 m. How to set fca to do the filtering? Thanks!
>>
>> Regards
>> Ed
>>
>> On Wed, Oct 28, 2015 at 10:20 AM, Dennis Shea <shea at ucar.edu> wrote:
>>
>>> A filter has no idea whether the data is temporal or spatial. Most
>>> filters do assume constant spacing. If your spatial data is
>>> irregularly spaced, you will likely have to interpolate to equal
>>> spacing.
>>>
>>> On Tue, Oct 27, 2015 at 2:51 PM, yang yang <yang.yang816 at gmail.com>
>>> wrote:
>>> >
>>> > In NCL, examples of Filters using filwgts_lanczos  are only in temporal
>>> > domain. Now I want to do low-pass for my 1-D spatial data such as
>>> dropsonde
>>> > data or flight-track data. Does NCL have a similar function as
>>> > filwgts_lanczos to do the work for 1-D spatial data? or How to apply
>>> > filwgts_lanczos in a 1-D spatial data? Thanks!
>>> >
>>> > Regards
>>> > Ed
>>> >
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>>
>>
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