TIME AXIS

[Juan Antonio Prego <japrego@xxxxxxxxxxx>]

    Hi,

    My machine is:

OPERATING SYSTEM: HP-UX hc0700 B.10.20 A 9000/770 2006886856 two-user license
FERRET         Version 4.45 - 01/17/97


    This is the problem:

    ERROR MESSAGE FROM FERRET:  COMMAND ---- >  use mask.nc (2-D VARIABLE)

 time:units = "hours since 1-1-1 0" ;

 **TMAP ERR: error in time line definition
             syntax error in "1-1-1 0



    This is the question?

    Should the time be coded using 0:0:0 instead of the single 0?
    i.e "hours since 1-1-1 0:0:0"



    Regards,
    J. A. Prego



    N. B. Additional information extracted from
http://www.met-office.gov.uk/sec5/CR_div/GDT_convention.html



24  Relative time

A time encoded as a relative time gives the elapsed interval since a specified
reference time; units takes the
form "time-unit since reference-time", as per the recommendations of the Unidata

udunits package (but
see below concerning the time-unit) e.g. a unit of seconds since 1992-10-8
15:15:42.5 indicates
seconds since 8 October 1992 at 3 hours, 15 minutes and 42.5 seconds in the
afternoon, in Universal
Coordinated Time (time zones can also be handled). In order to decode the values

on a relative time axis, the
application will in general need to know the calendar; the encoded time values
are meaningless without this
knowledge. Furthermore, a given date may result in different time values when
encoded in two different
calendars with the same units. For instance, 1996-2-1 15:00:00 is 62.625 days
since 1995-12-1
0:0:0 in the standard calendar, and 60.625 days since 1995-12-1 0:0:0 in the
360-day calendar.

The file udunits.dat defines second, minute, hour and day as units of time.
Units of months and years are
disallowed by Appendix C of this standard, because they are not well-defined;
since udunits defines a year as a
``tropical year'' of 31556925.97 s (674.03 s less than 365 days) and a month as
exactly a twelfth of a year, use
of these units will probably not give the expected results. For example, 1 month

since 1995-4-1 0:0:0
is treated by udunits as 30.4368 days since 1995-4-1 0:0:0, which is
approximately 1995-5-1
10:29, not 1995-5-1 0:0:0. Also, 1 year since 1995-4-1 0:0:0 is about 1996-3-31
5:49, not
1996-4-1 0:0:0. The udunits unit common_year (exactly 365 days) is permitted,
but not recommended.


A relative time axis for instantaneous measurements of a quantity: Measurements
are made at noon on
2nd-5th June 1996.

  dimensions:
    time=4;
  variables:
    double time(time);
      time:long_name="time";
      time:units="days since 1996-1-1 0:0:0";
  data:
    time=1.5, 2.5, 3.5, 4.5;