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zonal means [message #11082]

Mon, 16 March 1998 00:00

Martin Schultz
Messages: 515
Registered: August 1997

Senior Member

Hi everyone,

this is part question, part answer - I just want to make sure there
is nothing wrong with this:

Q: How do you compute zonal means from a 3D data cube ?
(example: A(72,46,14) is a data array with longitude, latitude, altitude
as dimensions, and I want to compute the averages over longitude for
each latitude and altitude)

A: well, you can do it in a loop (buuuuuhh!!)
for j=0,13 do begin
for i=0,45 do begin
b(i,j) = total(a(*,i,j)) / 72.
endfor
endfor

Q: but I hate loops !!

A: hmmm, we could try REBIN or CONGRID. How about that:
c = reform(rebin(a,1,46,14),46,14)
d = reform(congrid(a,1,46,14),46,14)

Q: That looks much more like IDL! But does it work ?

A: (after running a test with random data) Seems like REBIN is
just what you are looking for. CONGRID somehow screws it up. BTW: I am
sure you want to know the timing of these routines:
loop: 1.3769450 seconds
rebin: 0.15734899 seconds
congrid: 0.085584044 seconds
These results are obtained by repeating each method 100 times.

Q: But can I be ascertained that REBIN does the job correctly ? And does
anyone have an idea why CONGRID screws up ?

Thanks,
Martin.

------------------------------------------------------------ -------
Dr. Martin Schultz
Department for Earth&Planetary Sciences, Harvard University
186 Pierce Hall, 29 Oxford St., Cambridge, MA-02138, USA

phone: (617)-496-8318
fax : (617)-495-4551

e-mail: mgs@io.harvard.edu
IDL-homepage: http://www-as.harvard.edu/people/staff/mgs/idl/
------------------------------------------------------------ -------

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[Message index]

		zonal means By: Martin Schultz on Mon, 16 March 1998 00:00
		Re: zonal means By: Craig Markwardt on Wed, 18 March 1998 00:00
		Re: zonal means By: Karsten Rodenacker on Wed, 18 March 1998 00:00
		Re: zonal means By: Karsten Rodenacker on Wed, 18 March 1998 00:00
		Re: zonal means By: Karsten Rodenacker on Wed, 18 March 1998 00:00
		Re: zonal means By: Karsten Rodenacker on Wed, 18 March 1998 00:00
		Re: zonal means By: Martin Schultz on Tue, 17 March 1998 00:00
		Re: zonal means By: Martin Schultz on Mon, 23 March 1998 00:00
		Re: zonal means By: James Tappin on Mon, 23 March 1998 00:00

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