Cyclic variables

• To: netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: Cyclic variables
• From: Harvey DAVIES <hld@xxxxxxxxxxxx>
• Date: Mon, 3 Jul 1995 17:11:43 +1000 (EST)

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

• Follow-Ups:
  • Re: Cyclic variables
    • From: Steve Hankin