Re: derivatives on an uneven grid

["Rolando S. Balotro" <rbalotro@xxxxxxxxxxxxxxx>]

Hello Ferreters

How do you control the contour interval (begin:end) of your plot?

For example, in plotting a bathymetric data using etopo,
if once is only concern on the ocean part you begin from
0m to an arbitrary -1000m depth.

Masking the land portion
maybe good but still on the color scale, it goes from  negative 
value(ocean) to 0(coast) to positive value (mountains). Obviously
positive values are not needed.

Thanks in advance

truly a ferret-fan

rolando s balotro


John Donners wrote:
> Hi Brent,
>
> some weeks ago I ran into the same problem. I wrote a ferret script to 
> calculate first and second derivatives on an unequal grid:
>
> let zet=z[gz=rho]
> let h1=zet-zet[k=@shf:-1]
> let h2=zet[k=@shf:1]-zet
>
> let a=(-1)/(h1+h1*h1/h2)
> let b=1/(h2+h2*h2/h1)
> let c=(-1)*(a+b)
> let rhoz=a*rho[k=@shf:-1]+b*rho[k=@shf:1]+c*rho
> let aa=2/(h1*h1+h1*h2)
> let bb=2/(h2*h2+h1*h2)
> let cc=(-1)*(aa+bb)
> let rhozz=aa*rho[k=@shf:-1]+bb*rho[k=@shf:1]+cc*rho
>
> The variables rho and rhozz are now the first and second derivatives (in
> the vertical direction)  of the variable rho. The variable h1 is
> the distance between the uppermost and the central 
> points. The variable h2 is the distance between the central and the 
> lowermost points. It can easily be seen that the centered difference 
> formulations will show up if h1=h2.
>
> Hopefully this answers your questions. If there are any more questions 
> I'll be happy to answer them.
>
> Regards,
>
> John
>
> ------------------------------------------
> John Donners
> Royal Netherlands Meteorological Institute
> Dept. Oceanographic Research
>
> P.O. Box 201
> Wilheminalaan 10
> 3730 AE  De Bilt
> tel. nr. : ++31 30 2206701
> fax. nr. : ++31 30 2202570
> e-mail   : donners@knmi.nl
> --------------------------------------------------------------
> Zie ook/see also: http://www.knmi.nl/maildisclaimer.html