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strange thing about the grid



Hello, there: I have this weird thing with my underlying grid, and could not
figure out why. Here is what happened:
I have a 52 year monthly mean data from NCEP reanalysis. The underlying t axis
is irregular and centered on the 15/16th of each month. I was trying to extract
a particular month from that long time series by doing
let mar = temp[l=3:624:12]
let apr = temp[l=4:624:12]
....

But, the problem is that I can only do ONE MONTH per time. If I had more than
one definition, the ferret would give me the same value/grid of apr as mar. Here
is an example:

yes? show grid mar
    GRID (G002)
 name       axis              # pts   start                end
 LON       LONGITUDE          144mr   0E                   2.5W
 LAT37_73  LATITUDE            37 r   0                    90N
 LEVEL     HEIGHT (millib      17 i-  10                   1000
 (AX006)   TIME                52 i   16-MAR-1948 12:00    16-MAR-1999 12:00
yes? show grid apr
    GRID (G002)
 name       axis              # pts   start                end
 LON       LONGITUDE          144mr   0E                   2.5W
 LAT37_73  LATITUDE            37 r   0                    90N
 LEVEL     HEIGHT (millib      17 i-  10                   1000
 (AX006)   TIME                52 i   16-MAR-1948 12:00    16-MAR-1999 12:00
yes? 

But if I do
yes? can var mar
yes? show grid apr
    GRID (G002)
 name       axis              # pts   start                end
 LON       LONGITUDE          144mr   0E                   2.5W
 LAT37_73  LATITUDE            37 r   0                    90N
 LEVEL     HEIGHT (millib      17 i-  10                   1000
 (AX005)   TIME                52 i   16-APR-1948 00:00    16-APR-1999 00:00

Now the definition for apr is correct, and values are fine, too. It seems that
ferret only can recognize one definition like that, the first one.

However, if I define a  regular monthly grid like
yes? define axis/t=15-jan-1948:15-dec-1999:30.44/units=days treg
yes? define grid/t=treg grdnew

and then regrid the data to this new evenly spaced grid

yes? let tnew = temp[g=grdnew]
yes? let mar = tnew[l=3:624:12]
yes? let apr = tnew[l=4:624:12]

Now it works fine:

yes? show grid mar,apr
    GRID (G002)
 name       axis              # pts   start                end
 LON       LONGITUDE          144mr   0E                   2.5W
 LAT37_73  LATITUDE            37 r   0                    90N
 LEVEL     HEIGHT (millib      17 i-  10                   1000
 (AX005)   TIME                52 r   15-MAR-1948 21:07    18-MAR-1999 03:50
    GRID (G004)
 name       axis              # pts   start                end
 LON       LONGITUDE          144mr   0E                   2.5W
 LAT37_73  LATITUDE            37 r   0                    90N
 LEVEL     HEIGHT (millib      17 i-  10                   1000
 (AX007)   TIME                52 r   15-APR-1948 07:40    17-APR-1999 14:24
yes? 

Any idea on what's going on?
Thanks for your thought.
Muyin


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