Custom Query (124 matches)

Currently time coordinates must be stored as a number with units that are parsable by the udunits library (cf section 4.4).

I propose that we also allow time coordinates to be expressed as W3C/ISO 8601 formatted Strings. (Example: 2007-03-29T12:00:00Z) This format is widely used internationally, it is a profile by the W3C of the ISO standard. This page has a summary of the format and more links:

​http://www.unidata.ucar.edu/projects/THREDDS/tech/interfaceSpec/NetcdfSubsetService.html#Reference

Reasons:

• A string representation is human readable (compare with "348472938 secs since 2007-03-29 12:00:00Z")
• Independence from the udunits library when needed.
• May be easier to use for some.

Proposed change:

1) Change section 4.4 to start with:

"Date coordinates must either be of type char, formatted according to the W3C/ISO 8601 Date format, or of numeric type with a mandatory units attribute containing a udunit compatible date unit string."

2) Add an appendix that describes the W3C/ISO 8601 format, or simply point to an external link.

Note: I find it clearer to use "date" to refer to "calendar date", and "time" to mean a "unit of time duration" (eg 1500 secs). It might be worth looking through our usage to see where this meaning may be unclear.

This is a proposed correction for an error in the documents, not a change to the standard. According to the rules, it will be accepted if no-one objects to it in the next three weeks.

The table in Appendix A incorrectly states that flag_values is a string attribute. This is inconsistent with CF 3.5, which says, "The flag_values attribute is the same type as the variable to which it is attached." This error was pointed out by John Stark in April 2007 ​http://www.cgd.ucar.edu/pipermail/cf-metadata/2007/001614.html and this month by Philippe Poilbarbe on the netCDF email list.

The correction required is as follows:

• Add "D for the type of the data variable" to the sentence "The Type values are S for string, N for numeric" before the table in Appendix A.

• Change the type from S to D for flag_values in the table.

• In the conformance document, add a section:

  3.5 Flags
  Requirements:
  The flag_values attribute must have the same type as the variable to which it is attached.
  

Jonathan

In example 7.9 of the convention, the variable time_bounds appears but is not needed because the climatology_bounds are defined. Note that there is no "bounds" attribute pointing to time_bounds, so they aren't really illegal, but they are unnecessary and therefore might be confusing.

Recommend delighting time_bounds from the example.