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Re: Ambiguous coordinates on T axis



Hello,
Thanks for writing.  That's certainly interesting.  I think
the main thing to learn is that the "ambiguous coordinates"
warning message merits investigation, and that it is always
best to use explicit limits on variables when they don't
share exactly the same grid.

Ansley

Xiaobiao Xu wrote:

> well, I just tried the line you post, and I got different result!!!
> xiaobiao
> ============================================================================
> yes? set mode diag
> yes? list/t="1-jan-1990":"31-dec-1990" a[l=@ave] - b[l=@ave]
>  dealloc  dynamic grid (G002)          NORMAL    NORMAL    NORMAL    TWEEK
>  getgrid EX#1     C:  5 I:    1    1  J:    1    1  K:    1    1  L:    1
> 1
>  *** NOTE: Ambiguous coordinates on T axis: A[L=@AVE] - B[L=@AVE]
>  allocate dynamic grid (G001)          NORMAL    NORMAL    NORMAL    TDAY
>  strip limits reconciliation : EX#1
>  eval    EX#1     C:  5 I: -111 -111  J: -111 -111  K: -111 -111  L:    1
> 365
>  strip --> A[T=01-JAN-199000:00:31-DEC-199000:00@AVE,D=0]
>  found   A        M:  3 I: -111 -111  J: -111 -111  K: -111 -111  L:    1
> 365
>  doing --> A[T=01-JAN-199000:00:31-DEC-199000:00@AVE,D=0]
>  strip --> B[T=01-JAN-199000:00:31-DEC-199000:00@AVE,D=0]
>  found   B        M:  4 I: -111 -111  J: -111 -111  K: -111 -111  L:    1
> 314
>  doing --> B[T=01-JAN-199000:00:31-DEC-199000:00@AVE,D=0]
>              VARIABLE : A[L=@AVE] - B[L=@AVE]
>              TIME     : 01-JAN-1990 00:00 to 31-DEC-1990 00:00
>           8.514
> ==============================================================================
>
> On Thu, 2 Oct 2003, Ansley Manke wrote:
> > Hi again Steve,
> > I take back the first part of my previous answer.  The calculation:
> >
> > yes? shade/T="01-JAN-1994":"12-DEC-1994" \
> >   aglt[D=1,L=@AVE]-aglt[D=2,L=@AVE]
> >
> > does not in general compute the average from 01-JAN-1994 to
> > 12-DEC-1994 of the two variables and subtract them.  You
> > always need to specify the region for each variable separately
> > in square brackets to get the calculation right, or regrid one dataset
> > to the time axis of the other before doing the computation
> >
> > yes? shade aglt[D=1,T="01-JAN-1994":"12-DEC-1994"@AVE]- \
> >     aglt[D=2,T="01-JAN-1994":"12-DEC-1994"@AVE]
> >
> > or
> > yes? let algt2 = algt[d=2,gt=algt[d=1]]
> > yes? shade/T="01-JAN-1994":"12-DEC-1994" \
> >      aglt[D=1,L=@AVE]-aglt2[L=@AVE]
> >
> > Try this: make up a couple of time series variables on two
> > different time axes, and use MODE DIAGNOSTIC to see what
> > Ferret makes of the calculation. Look at the lines that start with
> > "doing -->" to see that the average for variable a is correct, but
> > the averaging for variable b does not have the right time range.
> > Ferret cannot make sense of the command.
> >
> > yes? def axis/t="1-jan-1990":"31-dec-1990":1/units=days tday
> > yes? def axis/t="1-jan-1989":"31-dec-1994":7/units=days tweek
> >
> > yes? let a = sin(t[gt=tday]/10) * t[gt=tday]/100
> > yes? let b = cos(t[gt=tweek]/20)* (1-t[gt=tweek]/100)
> >
> > yes? plot a   ! Plot the variables to load them into memory
> > yes? plot b
> >
> > yes? set mode diag
> > yes? list/t="1-jan-1990":"31-dec-1990" a[l=@ave] - b[l=@ave]
> >
> >  dealloc  dynamic grid (G002)          NORMAL    NORMAL    NORMAL    TWEEK
> >  getgrid EX#1     C:  5 I:    1    1  J:    1    1  K:    1    1  L:    1    1
> >  *** NOTE: Ambiguous coordinates on T axis: A[L=@AVE] - B[L=@AVE]
> >  allocate dynamic grid (G001)          NORMAL    NORMAL    NORMAL    TDAY
> >  strip limits reconciliation : EX#1
> >  eval    EX#1     C:  5 I: -111 -111  J: -111 -111  K: -111 -111  L:    1  365
> >  strip --> A[T=01-JAN-199000:00:31-DEC-199000:00@AVE,D=0]
> >  found   A        M:  3 I: -111 -111  J: -111 -111  K: -111 -111  L:    1  365
> >  doing --> A[T=01-JAN-199000:00:31-DEC-199000:00@AVE,D=0]
> >  strip --> B[T=01-JAN-198900:00:31-DEC-198900:00@AVE,D=0]
> >  found   B        M:  4 I: -111 -111  J: -111 -111  K: -111 -111  L:    1  314
> >  doing --> B[T=01-JAN-198900:00:31-DEC-198900:00@AVE,D=0]
> >              VARIABLE : A[L=@AVE] - B[L=@AVE]
> >              TIME     : 01-JAN-1990 00:00 to 31-DEC-1990 00:00
> >          -0.1186
> >
> >
> >
> > Ansley Manke wrote:
> >
> > > Hi Steve,
> > > When you do a calculation over time when there are two different
> > > time axes involved in the expression, Ferret gives this informational
> > > message just to let you know that the two time averages are different.
> > > It's using each variable's time axis and is doing the calculation you
> > > expected. You could make it absolutely clear what you're doing by
> > > specifying the time interval as a qualifier for each variable, in square
> > > brackets.  (I use the backslash continuation character here)
> > >
> > > yes? shade aglt[D=1,T="01-JAN-1994":"12-DEC-1994"@AVE]- \
> > > aglt[D=2,T="01-JAN-1994":"12-DEC-1994"@AVE]
> > >
> > > See "region" in the Users Guide index; this is the third way to specify
> > > a region and is almost always the best way to be sure you're doing
> > > what you think you're doing. And, this is also the way to do it if you
> > > want to compare averages over two different time intervals:
> > >
> > > yes? shade aglt[D=1,T="01-Jan-1994":"12-Jun-1994"@AVE]- \
> > > aglt[D=2,T="13-Jun-1994":"12-Dec-1994"@AVE]
> > >
> > > Ansley Manke
> > >
> > > Steve Knox wrote:
> > >
> > > > How can I avoid this ambiguity on the time axis when I'm dealing with 2
> > > > datasets?
> > > > I guess it's confused since it dosen't know which T axis to average over.
> > > > In this case the time periods are the same so the ambiguity doesn't
> > > > matter, but if I want
> > > > to specify  different time periods in each dataset how can I do this?
> > > >
> > > > yes? use agltHistoric.cdf
> > > > yes? use agltCENT.cdf
> > > > yes? shade/T="01-JAN-1994":"12-DEC-1994" aglt[D=1,L=@AVE]-aglt[D=2,L=@AVE]
> > > >
> > > > *** NOTE: Ambiguous coordinates on T axis: AGLT[D=1,L=@AVE]-AGLT[D=2,L=@AVE]
> > > >
> > > > Thanks for any help on this.
> > > >
> > > > Steve Knox
> > > > NREL
> > > > Colorado State University
> > >
> > > --
> > > Ansley Manke  Pacific Marine Environmental Laboratory  Seattle WA
> >
> > --
> > Ansley Manke  Pacific Marine Environmental Laboratory  Seattle WA
> >
> >
>
>  xiaobiao xu
>  MPO/RSMAS UM, 4600 Rickenbacker Csw.
>  Miami,FL,33149
>  Phone:(305)-361-4628(O); 595-7844(H)
>  E-mail:xbxu@rsmas.miami.edu

--
Ansley Manke  Pacific Marine Environmental Laboratory  Seattle WA




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