Re: subsampling of grided data onto point location and time.

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Yogesh,
To find the value of CO2_mod which matches a location from your
CO2_sat data set,  assuming CO2_mod is data set 1 and CO2_sat
is data set 2, you can define variables based on data set 2 and use
them to choose values from data set 1.  For example if you want the
value of CO2_mod at the longitude, latitude location of the15th of
the second data set,

  let lon_x = lon[d=2,L=15]
  let lat_y = lat[d=2, L=15]
  let sampled_data = CO2_mod[d=1,x=`lon_x`, y=`lat_y`]

The grave accents (or back-tic character) force the evaluation
of lon_x and lat_y to single scalar values.

This would give you CO2_mod at one location, and at all of its
original 100 time steps. You can specify a particular time in the
range of the time steps using one or more date strings or time steps
between 1 and 100, for example, if your data includes dates in 2003,
then

 list sampled_data[t="12-jan-2003:12:00"] 
 list sampled_data[L=1:5]

If you want to sample your gridded data at many point locations,
you can use the SAMPLEXY function.  To get CO2_mod at the first
1000 the locations in the second data set, and a single date in the
first data set,

  let sampled_data = SAMPLEXY \
     (CO2_mod[d=1,t="12-jan-2003:12:00"], \
       lon[d=2,L=1000], lat[d=2,L=1:1000])

This will result in 1000 values of CO2_mod. (I have broken up
the command line into 3 lines using the continuation character,
the backslash so it is easier to read.)

Now you can compare the two CO2 data sets:

  let sampled_data = SAMPLEXY \
     (CO2_mod[d=1,t="12-jan-2003:12:00"], \
       lon[d=2,L=1000], lat[d=2,L=1:1000])

  plot/L=1:1000 CO2_sat - sampled_data


Yogesh K. Tiwari wrote:

> Hello Ferret Users,
>
> How I can do the subsampling of a grided data on a given point 
> location and time.
>
> Both the grided data and point data are in netcdf format.
>
> The grided data looks like, for example :
>
> title         I        J      K        L
> CO2_mod     1:360    1:180   --      1:100
>
> here L is time steps.
>
> And point data looks like :
>
> title         I        J      K        L
> CO2_sat      --       --      --    1:1860206
> lat          --       --      --    1:1860206
> lon          --       --      --    1:1860206
>
> here L is number of index.
>
> I could not get much help from ferret users guide.
>
> Many thanks in advance,
>
> Regards,
> Yogesh