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Re: Gradient of two dimensional field [message #8294 is a reply to message #8200]

Wed, 19 February 1997 00:00

bowman
Messages: 121
Registered: September 1991

Senior Member

In article <brian.jackel.106.19DE53E0@uwo.ca>, brian.jackel@uwo.ca (Brian
Jackel) wrote:

>>> we want to determine the electrical field from a given potential,
>>> i.e. we have to calculate the gradient of a two dimensional array.
>>>
>>> Has anybody a idl-pvwave procedure to do this task?
>
>> I would think that the shift function (used twice)
>> could be used to do this.
>
> Or even just
>
> dx= a(1:*,*) - a
> dy= a(*,1:*) - a
>
> or
>
> dx= a(1:n-1,*) - a(0:n-2,*)
> dy= a(*,1:m-1) - a(*,0:m-2)
>
> if "a" has dimensions of (n,m). When doing it the first way IDL takes
> care of the different array sizes, with no perceptible performance hit.
> The second way is perhaps a bit easier to read. Is this what you (the
> original poster) were after?

You may want to use centered differences, i.e.

dzdx = (SHIFT(z,-1, 0) - SHIFT(z, 1, 0))/(2.0*dx)
dzdy = (SHIFT(z, 0,-1) - SHIFT(z, 0, 1))/(2.0*dy)

(I trust the compiler is smart enough to convert the division to
multiplication.)

Don't forget to fix the edges, i.e., use uncentered differences for the
normal component or whatever is appropriate for your problem.

Ken

--
Kenneth P. Bowman, Assoc. Prof. 409-862-4060
Department of Meteorology 409-862-4132 fax
Texas A&M University bowman@csrp.tamu.edu
College Station, TX 77843-3150
Satellite ozone movies on CD-ROM --> http://www.lenticular.com/

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[Message index]

		Gradient of two dimensional field By: Wilpert_Martin on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: Andy Loughe on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: davidf on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: stockwell on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: Andy Loughe on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: bowman on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: brian.jackel on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: Mirko Vukovic on Wed, 19 February 1997 00:00
		Re: Gradient of two dimensional field By: Andy Loughe on Mon, 03 March 1997 00:00

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