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Re: [ferret_users] modulo longitude standards?

On 2/9/2016 4:47 PM, Ryo Furue wrote:
Hi Steve,

Thanks for your detailed explanation.

An axis may be less than global (e.g. axis "x30") and still be understood to
have modulo 360 behavior.  This is inferred automatically from its units of
degrees_east.   To reliably regard a variable defined on an X axis of more
than 360 degrees, Ferret would have to check that the overlapping points at
each end all had identical values.
Or you COULD just issue a warning and disregard the values on one of
the overlapping "layers".  I'm not saying you SHOULD.

  Ferret has no internal logic to do this.
Yes, I guessed that that is the reason why Ferret currently rejects
such an overlapping axis.

There are cases where people deliberately create variables on longitude ranges of (say) 720 degrees (double worlds). All other things being equal it seems best not to second guess what people are doing.

P.S.  The attribute "modulo" is from older Ferret versions.
So, what's the latest alternative?  Is it a Ferret-specific attribute?
  Or is it part of a well-recognized convention like CF?
The "modulo" attribute was a Ferret creation -- not CF. Somewhere along the way, Ferret was enhanced to support the notion of a "subspan modulo axis" -- modulo behavior, even though the axis isn't globe-encircling. The axis X30 in the previous message was an example. With the advent of subspan modulo axes, all longitude axes 360 degrees or less became implicitly "modulo". So the older modulo attribute is essentially irrelevant. (But no harm from having it in a file AFAIK.)

    - Steve
The dataset I was given actually includes the "modulo" attribute.  I'm
not sure if the creators of the dataset use Ferret.

Basically, I'd like to know if I should ask the creators to fix it.
They are a data center and so they do care about these things.


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