Re: Some questions about function FFTA and FFTP

["Dr.Sudheer Joseph" <sjo@xxxxxxxxxxxxx>]

Dear DuHui,
            The default unit used in ferret is cycles/day( in example with
navy winds) if your data is sampled at hours ,  you will get the axis values
in cycles per hour provided in  your netcdf file the definition of time
units is "hours". If you are using ascii file,define taxis units as hours or
days or months which ever you want. Once this is done unit problem is
solved.
example:
for a data sapled at every 24 hour from 2001 Jan to 2002 Jan

 define axis/t=1-jan2001:1-jan-2002:24/units=hours tax
 file/var=CO2  co2.dat
 let tco2=co2[gt=tax]
 let fftco2=ffta(tco2)
 plot fftco2


question NO.2  Box sixe is nothing but your total time length since ferret
treats every thing in terms of grid this terminology is used.

Niquist frequency: this is the highest frequency you can obtain from the
data you are using with a specified sampling interval  which is
1/2*N(del_T), where N is the total number of samples del_T is the sampling
interval(Total time=N*del_T) Boxsixe= N*det_t because ferret has already
taken in to account the time units.
Hope this helps.

Sudheer
****************************************************************
Dr.Sudheer Joseph
Scientist- 'B'
Indian National Centre for Ocean Information Services
Plot No# 3, Nandagiri Hills Layout, Jubilee Hills,
Hyderabad, India, Pin:500 033.
Ph: +91- 040-23553542/43/ (O); +91-040 23044600 (R) ;
Fax:+91-040-3551096
E-mail:sjo@rediffmail.com;callsjo@yahoo.com
*****************************************************************
"The ultimate measure of a man is
not where he stands in moments of
comfort and convenience, but where
he stands at times of challenge and
controversy."
                        Martin Luther King, Jr.
************************************************************

----- Original Message -----
From: "du hui" <bjduhui@hotmail.com>
To: <oar.pmel.ferret_users@noaa.gov>
Sent: Wednesday, May 26, 2004 2:34 PM
Subject: Some questions about function FFTA and FFTP


> Dear ferret users:
>  I am using FFTA to do some research. I just want to use different units
of
> T axis.
> I have read the user's guide. But I can not understand them well.
>   In the ferret guides , there is the following description about FFTA
>
> "The units of the returned time axis are "cycles/Dt"  where Dt is the time
> unit of the input axis.  The Nyquist frequency is yquist =
1./(2.*boxsize),
> and the frequency axis runs from freq1 = yquist/ float(nfreq) to freqn =
> yquist "
>
> question 1.
>  the returned time axis should be day because the time unit of my input
> axis is day. However the actual returned time axis is hour.
> question 2
> what does Nyquist frequence mean? What is the boxsize?
>
> actually I am not very clear to the process of converting the time axis
> into what I need. For example, from cycle/day to cycle/month
> ok ,that is the problme
> Thanks a lot
>
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