Re: [ferret_users] The @DDC transformation in contour plots

[Jaison Kurian <jaison@xxxxxxxxxxxxxxxxxx>]

Hi Kirsty,
            The @DDC transformation on time axis will not eliminate
the time dimension. You will have exactly the same number of time points
as you started with. However, the end points will be empty as DDC 
computes the centered derivative. For more information, please have a look
at User Manual, Ch3 Sec2.4.16.  @DDC?centered derivative. Here is a simple
example.

  yes? use monthly_navy_winds
  yes? list/x=60/y=10 uwnd[l=1:10@DDC]
             VARIABLE : ZONAL WIND (M/S)
                        centered derivative on T
             FILENAME : monthly_navy_winds.cdf
             FILEPATH : /home/jaison/DATA/ferret_dsets/data/
             SUBSET   : 10 points (TIME)
             LONGITUDE: 60E
             LATITUDE : 10N
                       60E
                       17
 16-JAN-1982 /  1:       ....
 16-FEB-1982 /  2:  3.082E-07
 18-MAR-1982 /  3:  5.025E-07
 18-APR-1982 /  4:  7.557E-07
 18-MAY-1982 /  5:  1.501E-06
 18-JUN-1982 /  6:  1.364E-06
 18-JUL-1982 /  7:  1.439E-07
 17-AUG-1982 /  8: -7.676E-07
 17-SEP-1982 /  9: -1.742E-06
 17-OCT-1982 / 10: -1.222E-06

Thus, the result of @DDC will have 3-dimensions(XYT) and hence you have
to fix 0ne of them to get a shaded/contoured plot. Please note that you can
use the following commands to get an idea about variable's grid/shape

  yes? show grid uwnd
    GRID GDN1
 name       axis              # pts   start                end
 FNOCX     LONGITUDE          144mr   20E                  17.5E(377.5)
 FNOCY     LATITUDE            73 r   90S                  90N
 normal    Z
 TIME      TIME               132 r   16-JAN-1982 20:00    17-DEC-1992 03:30

  yes? say `uwnd,return=shape`
 !-> MESSAGE/CONTINUE XYT
XYT

Please let me know if you have any questions.

Jaison

On Tue, 22 Jan 2008 15:33:20 +0000, Kirsty Hanley wrote
> Hi,
> 
> I'm using Ferret v5.81 on Solaris. I have a NetCDF file containing a 
> variable, swh, with X=1:120, Y=1:240 at 6 hourly timesteps for 1 
> year. I want to calculate and produce a contour plot of the time 
> derivative of this variable at each point in space. I tried using 
> the @DDC command:
> 
> yes? fill swh[l=2:28@DDC]
> 
> but received the following error:
>  **ERROR: dimensions improperly specified: must be a 2D region
> 
> If I specify a value of X and Y I can produce a line plot of the derivative:
> 
> yes? plot/i=60/j=60 swh[l=2:28@DDC]
> 
> Can the @DDC command only be used at a single point? If so, how can 
> I calculate the time derivative over the entire region?
> 
> Any help much appreciated,
> Kirsty
> 
> -- 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Kirsty Hanley, PhD student
> Room 2U06, Department of Meteorology,
> University of Reading, Earley Gate
> PO Box 243,
> READING. RG6 6BB. U.K. 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
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