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Re: Sigma coordinates and Ferret

Hi Donald,

You'll find a write-up dealing with sigma coordinates in the Frequently Asked
Questions document. (You'll find it from the Ferret home page at

In that document under "Data Management"/"Miscellaneous" you'll find

	"How do I handle sigma coordinate output in Ferret?"

This write-up may answer your questions. Feel free to ask further if this
doesn't have what you're after.

	- steve


On Dec 3,  7:38am, Donald S. Dunbar wrote:
> Subject: Sigma coordinates
> I am working with 4D data sets (X,Y,Z,T) where the vertical axis Z is
> defined as Z = (Z'+E)/(H+E); -E<=Z<=H; E,H = elevation,depth.  Z' is the
> depth below LLW and Z: 0<=Z<=1 is the sigma coordinate.  I want to
> convert data from Z to Z'.  To date I have been doing the following (at
> 10 m, for example) for a 4D variable Q and a vector of SIGMA values:
> let ZP = (H+E)*SIGMA - E   ..... define the depth below LLW
> let MASK = ZP[Z=@WEQ:10]   ..... find where ZP = 10 m
> let TMP = Q*MASK           ..... A matrix of weighted Q values
> let Q_10 = TMP[Z=@SUM]     ..... Q at 10 m
> Then I repeat the procedure to get Q_20, Q_30, etc.  All of this is in a
> GO file generated by a PERL script (so it is automated somewhat). I then
> would like to combine all of the resulting horizontal slabs back into a
> 4D array which will have a fixed vertical spacing (10 m in this
> example).
> My question is: Is all of this necessary, and isn't there an easier way?
> Sigma coordinate systems are common enough that I would think some nice
> transformation would be built in.  I don't see any reference in my
> manual, however.
> If there is no other way, what is the easiest way to stack the slabs
> into a 4D array?  Say I have a new grid GQ given by:
> How do I define a variable on GQ which consists of the slabs Q_10, etc.
> determined above?
> Thanks for any pointers.
> --
> ------------------------------------------------------
> D.S. Dunbar (dsd@oar.com)  Ocean Applied Research Ltd.
> 1864 Duchess Ave.  West Vancouver BC  V7V 1R1  CANADA
> ------------------------------------------------------
>-- End of excerpt from Donald S. Dunbar


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		|  Seattle, WA 98115-0070  |  hankin@pmel.noaa.gov

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