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Re: [ferret_users] How to do extrapolation in ferret



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



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