Problem with calculating derivatives

[Glenn Carver <Glenn.Carver@xxxxxxxxxxxxxxxx>]

I'm having problems trying to get Ferret to compute some vertical
derivatives. I'm getting some odd results that I cannot fathom.

I have a grid, called 'ngh'. I define a variable like this:

let zz=z[g=ngh]-z[g=ngh]+k
list/k=1:4 zz
             Z[G=NGH]-Z[G=NGH]+K
 330    /  1:   1.00000
 358    /  2:   2.00000
 388.4  /  3:   3.00000
 421.3  /  4:   4.00000

I want to compute the vertical derivative of this so I do:

list/k=1:4 zz[z=@ddc]
             Z[G=NGH]-Z[G=NGH]+K
             centered derivative on Z
 330    /  1:      ....
 358    /  2:  1.349215
 388.4  /  3:  1.243527
 421.3  /  4:  1.146478

I can't see where on earth Ferret gets these numbers from. A centred
derivative ought to be (and the manual says this too):
   ( zz(k+1) - zz(k-1) ) / ( z(k+1) - z(k-1) )

For k=2 that ought to be :   ( 3 - 1 ) / ( 388.4 - 330 ) = 0.00342 ???

I have no idea where Ferret gets 1.349215 from. What's going on??

   Thanks
             Glenn


----
Dr. Glenn Carver, Senior Research Associate,
Centre for Atmospheric Science, Chemistry Dept., Cambridge University, UK
mailto:Glenn.Carver@atm.ch.cam.ac.uk   http://www.atm.ch.cam.ac.uk/~glenn/

              "I never think of the future, it comes soon enough"
                   - Albert Einstein