Some variables are cyclic.
A common example is longitude which has a cycle of 360 degrees. This means
that (assuming unit="degrees_east") one should treat a value of 360 as
equivalent to 0, -180 as equivalent to 180, etc. In other words the value
should be treated as modulo 360.
How should such variables be handled by netCDF?
I propose that:
1. There be a new standard conventional attribute called "cycle_length",
(or perhaps "modulus") giving the cycle length (in the units defined by
attribute "units").
2. The unit "degrees_east" (and equivalents degree_east, degreeE, degree_E,
degreesE, degrees_E) should imply cycle_length=360.
Note that I consider it better to identify longitude from the unit rather
than the variable name (which could be "lon", "longitude", etc.)
One could consider 'cyclicity' to be a property of the unit. It would be
possible to include in the units database file ('udunits.dat') a new field
giving the cycle, if any. However I am inclined to feel this is not really
necessary and would be more trouble than it is worth. I am not aware of any
common units other than "degrees_east" which should always be treated as
cyclic, although there may well be some.
Harvey Davies, Home: +61 3 9772 5199
CSIRO Division of Atmospheric Research, Work: +61 3 9586 7574
Private Bag No. 1, Mordialloc, Fax: +61 3 9586 7600
Victoria 3195, Australia E-mail: hld@xxxxxxxxxxxx
