Re: [ferret_users] regridding transformation should reduce dimension . . .

[Andrew Wittenberg - NOAA Federal <andrew.wittenberg@xxxxxxxx>]

Hi Ryo,

Thanks for the clearly-stated question.  [All: this is a good example
of how to ask a question so that it gets answered quickly.  In
particular, Ryo provided a working example using one of Ferret's
built-in datasets, e.g. levitus_climatology, coads_climatology, or
monthly_navy_winds; and also pasted in the Ferret header, which shows
what Ferret version he was using, just in case it turned out to be a
bug.]

Note that Z=@LOC doesn't mean "search from top to bottom for 10 degC."
 Instead it means "search within the future evaluation context for 10
degC."

So when you later specify z10[k=1], @LOC dutifully searches only over
k=1; and since 10degC isn't found in the top level, it evaluates to
"missing."

To insulate @LOC from future contexts, specify explicit z-limits:

  NOAA/PMEL TMAP
  FERRET v6.951 (10/29/2015)
  Linux 2.6.32-573.7.1.el6.x86_64 64-bit - 10/30/15
  1-Dec-15 12:40

yes? use levitus_climatology
yes? let z10 = temp[z=0:6000@LOC:10]
yes? let below_z10 = z[g=temp] LT z10
yes? shade/x=180 below_z10
yes? shade/z=400 below_z10

The relevant documentation is:

http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/variables-xpressions/XPRESSIONS#_VPINDEXENTRY_528

See also the following answer in the Ferret email archives:

http://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2006/msg00098.html

which was found by searching the email archives for "missing context".

Andrew


On Tue, Dec 1, 2015 at 6:06 AM, Ryo Furue <furue@xxxxxxxxxx> wrote:
> Hi Ferret users,
>
> I'm sure there has been discussions about this issue and I searched
> our email archive and found some threads that touch upon it, but
> didn't find whether it's "solved" or not.
>
> The problem is that @LOC (and perhaps other similar transformations)
> doesn't properly "eliminate" the axis.  For a (x,y,z) temperature
> field, say, you can find the depth of an isopycnal:
>
>    yes? z10 = temp[z=@LOC:10]
>
> but, "SHOW GRID z10", it still has more than one gridpoints.  Nevertheless,
>
>     yes? fill z10[k=1]   !--> undefined field
>     yes? fill z10           !--> works!
>
> See a complete example below.  This behavior looks like a bug to me.
>
> This matters.  I wanted to define a mask on the basis of this
> variable, something like:
>
>    yes? let below_z10 = if z[g=temp] lt z10 then 1
>
> but this doesn't work because z10[k=1, 2, 3, . . .] are undefined!
>
> My temporary solution is
>
>    yes? let grid = x[gx=temp] + y[gy=temp]
>    yes? let z10_squashed = reshape(z10,grid)
>    yes? let below_z10 = if z[g=temp] lt z10_squashed then 1
>
> But, I guess Ferret should do this reshaping for me with @LOC.
>
> Regards,
>
> Ryo
> ----------------------------------------
> $ ferret
>   NOAA/PMEL TMAP
>   FERRET v6.94
>   Darwin 14.1.0 - 02/26/15
>   1-Dec-15 19:36
>
> yes? set data levitus_climatology
> yes? let z10 = temp[z=@LOC:10]
> yes? show grid z10
>     GRID GMS1
>  name       axis              # pts   start                end
>  XAXLEVITR LONGITUDE          360mr   20.5E                19.5E(379.5)
>  YAXLEVITR LATITUDE           180 r   89.5S                89.5N
>  ZAXLEVITR DEPTH (m)           20 i-  0                    5000
>  normal    T
>  normal    E
>  normal    F
> yes? list/x=180/y=0 z10[k=1]
>              VARIABLE : TEMP[Z=@LOC:10]
>              FILENAME : levitus_climatology.cdf
>              FILEPATH : /usr/local/ferret/data/
>              LONGITUDE: 179.5E
>              LATITUDE : 0.5S
>              DEPTH (m): 0 to 5
>         ....
> yes? list/x=180/y=0 z10
>              VARIABLE : TEMP[Z=@LOC:10]
>              FILENAME : levitus_climatology.cdf
>              FILEPATH : /usr/local/ferret/data/
>              LONGITUDE: 179.5E
>              LATITUDE : 0.5S
>              DEPTH (m): 0 to 5000
>           371.7
> !---- temporary solution ----
> yes? let grid = x[gx=temp] + y[gy=temp]
> yes? let z10_reshaped = reshape(z10, grid)
> yes? show grid z10_reshaped
>     GRID (G002)
>  name       axis              # pts   start                end
>  XAXLEVITR LONGITUDE          360mr   20.5E                19.5E(379.5)
>  YAXLEVITR LATITUDE           180 r   89.5S                89.5N
>  normal    Z
>  normal    T
>  normal    E
>  normal    F
> yes? list/x=180/y=0 z10_reshaped[k=1]
>              VARIABLE : RESHAPE(Z10, GRID)
>              FILENAME : levitus_climatology.cdf
>              FILEPATH : /usr/local/ferret/data/
>              LONGITUDE: 179.5E
>              LATITUDE : 0.5S
>           371.7
> yes? list/x=180/y=0 z10_reshaped
>              VARIABLE : RESHAPE(Z10, GRID)
>              FILENAME : levitus_climatology.cdf
>              FILEPATH : /usr/local/ferret/data/
>              LONGITUDE: 179.5E
>              LATITUDE : 0.5S
>           371.7
> yes?