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Re: derivatives on an uneven grid



Hi Rolando,
To control the contour interval of the plot you need the
CONTOUR/LEVELS qualifier.  In the Users Guide,
see "levels, contour; general discussion" for the many options
that are available.

Ansley Manke


"Rolando S. Balotro" wrote:

> 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
> >

--
Ansley Manke  Pacific Marine Environmental Laboratory  Seattle WA  (206)526-6246




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