**To**:**"Donald S. Dunbar" <dsd@xxxxxxx>****Subject**:**Re: Sigma coordinates and Ferret****From**:**hankin@xxxxxxxxxxxxxxxxxxx (Steve Hankin)**- Date: Tue, 3 Dec 1996 09:11:01 -0800
- Cc: ferret_users@xxxxxxxxxxxxxxxxxxx
- In-reply-to: "Donald S. Dunbar" <dsd@oar.com> "Sigma coordinates" (Dec 3, 7:38am)
- References: <32A44980.6FA2@oar.com>
- Sender: owner-ferret_users

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 http://www.pmel.noaa.gov/ferret/home.html). 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: > > DEFINE GRID/LIKE=Q/Z=10:100:10 GQ > > 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 -- | NOAA/PMEL | ph. (206) 526-6080 Steve Hankin | 7600 Sand Point Way NE | FAX (206) 526-6744 | Seattle, WA 98115-0070 | hankin@pmel.noaa.gov

**References**:**Sigma coordinates***From:*Donald S. Dunbar

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