[CF-metadata] How to define time coordinate in GPS?

Timothy Patterson Timothy.Patterson at eumetsat.int
Mon Jun 29 04:16:11 MDT 2015


I understand that for climate or forecast data that 30+ seconds of inaccuracy may not be significant, but even though the C and F in "CF Conventions" stand for "Climate and Forecast", the conventions are also being increasingly adopted for instrument and measurement datasets. In these cases, time accuracy at small time scales becomes more important, which is why seeing a proposed convention that allows the time to be written ambiguously (so that there may or may not be discontinuities or offsets) is rather disconcerting, like telling a climate scientist that the netCDF encoding of his temperature data may or may not have introduced an inaccuracy of a few Kelvin in some readings.  

Under the CF1.6 conventions, I believe the base time or epoch time was always expressed in UTC and the calendar attribute was applied to the encoded time count. Using this convention, I could specify a UTC start time and then have a simple GPS-like count of elapsed seconds (with no discontinuities introduced by leap seconds). Under the proposals for the 1.7 convention, this doesn't seem possible. The epoch and time count must both be expressed either as UTC or as GPS time and the only "mixed calendar" options introduce the above mentioned ambiguity.

Regards,

Tim Patterson


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