Re: [ferret_users] How to do extrapolation in ferret

[Akshay Hegde <akshay.k.hegde@xxxxxxxxx>]

Thanks Martin, for explanation regarding extrapolation tool.


Akshay

On Thu, Mar 21, 2013 at 8:50 PM, Martin Schmidt
<martin.schmidt@xxxxxxxxxxxxxxxxx> wrote:
> Akshay,
>
> fill_xy is a horizontal extrapolation tool. There is no xz, yz or xt
> counterpart yet.
>
> The method makes explicit use of the fact, that a grid point has neighbours
> in
> the x-y-plane. It is not suitable for line data.
>
> However, extrapolating data is generally risky. I means basically, to invent
> data
> from some assumptions. There is no program, that can do this step for you.
>
> For example:
>
> generate a vertical grid and some artificial data:
>
> define axis/depth/z=0:100:2/unit=m zax
> define grid/z=zax z_grid
> ! some data in the uppwer 50 m
> let a = if z[gz=z_grid] lt 50 then 20-0.1*z[gz=z_grid] else 1/0
> plot/hlimit=0:20 a
>
> ! This is an example data set. Feel free to replace it by your real data.
> Please see the ferret manual, how to read
> ! ASCII data from file.
> ! How to extrapolate now? May be you have a well mixed bottom layer.
> ! Use the nearest valid point below 50 m.
> plot/hlimit=0:100/over a[z=@fnr]
>
> ! No mixing near the bottom? Better to extrapolate (May be there is another
> solution)
> ! Use the value at z=49 and the backward derivative.
> let b = missing(a,-1)
> let c= if b lt 0 then a[z=49] + a[z=49@ddb]*a[z=@cdb] else a
> plot/over c
>
> Any other formula for c is also correct, because nothing is really known
> below 50 m depth.
> For sure some assumptions are more likely some are less likely, some are not
> justified.
>
> Finally - this problem is independent off the format of the input data. You
> may read your data from file
> in ASCII, netcdf, binary. Once read the data, the problem is the same in any
> case.
>
> Hope this helps a little bit, Greetings,
> Martin
>
>
> Akshay Hegde wrote:
>>
>> Hi ! all
>>
>> Interpolation can be done easily in ferret by regridding
>> transformation, suppose if extrapolation has to be performed for ascii
>> file what is the procedure, I had gone through following link for
>> gridded datasets, and I have no I idea that whether it's possible in
>> ferret for ascii datasets
>>
>>
>>
>> http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/appendix-a-external-functions/fill_xy
>>
>> My requirement is like this assume that ascii file contains
>> temperature and and depth data of one location profile maximum depth
>> is 50 meter bin size is 1 meter, suppose if we have to extend till 65
>> meter, what we need to do ?
>>
>> those who know Kindly Help.
>>
>>
>> Thanks in advance,
>>
>> Akshay
>>
>
>