| Re: IDL 5.4 label_date function [message #27791] |
Wed, 07 November 2001 14:54 |
Wayne Landsman
Messages: 117
Registered: January 1997
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Senior Member |
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> This would have made sense 2000 years ago when the date changed at midday,
> but these days it only makes sense to IDL developers.
>
Well, it also once made a small amount of sense to (non-solar) astronomers,
since starting at noon meant that telescope observations taken over one night
would all have the same integral value of Julian date (JD). Of course, this
is irrelevant for telescope observations from space , or for Australia/New
Zealand/Japan where noon UT (Greenwich time) occurs in the middle of the
night. So even astronomers now typically use modified Julian date (MJD)
defined as JD- 2400000.5 and which does begin at midnite.
Wayne Landsman landsman@mpb.gsfc.nasa.gov
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| Re: IDL 5.4 label_date function [message #27792 is a reply to message #27791] |
Wed, 07 November 2001 14:37  |
Chad Bahrmann
Messages: 7
Registered: September 2001
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Junior Member |
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Thanks to all for the information....I guess I really did not fully
understand julday
Chad
"Chad Bahrmann" <cbahrmann@ou.edu> wrote in message
news:QscG7.6556$o4.12893@news.ou.edu...
> Has anyone encountered problems in using the label_date function in IDL
5.4?
> From the documentation for IDL 5.4. This plot gives you data from
> 11/01/2001 at 1200 through 11/02/2001 at 1200 when I would expect data
from
> 11/01/2001 at 0000 through 11/02/2001 at 0000. I have had success using
> this function in older versions of IDL but this one appears not to work
> properly? The actual code had revisions on
> ; MODIFICATION HISTORY:
> ; DMS, RSI. April, 1993. Written.
> ; DMS, RSI. March, 1997. Added Time format.
> ; DMS, RSI. Jan, 1999. Rounded least significant time unit
> ; CT, RSI. May 2000. Completely rewrote to use calendar format codes.
> ; Added Level argument for new date/time TICKUNITS.
> ; Added AM_PM and DAYS_OF_WEEK keywords, '%A' and '%W'
codes.
> ;-
>
> PRO test
>
> ; Create format strings for a two-level axis:
>
> dummy = LABEL_DATE(DATE_FORMAT=['%H%I!C%N/%D/%Y'])
>
> ;Generate the Date/Time data
> time = TIMEGEN(START=JULDAY(11,1,2001), FINAL=JULDAY(11,2,2001))
>
> ;Generate the Y-axis data
> data = RANDOMN(seed, N_ELEMENTS(time))
>
> ;Plot the data
> PLOT, time, data, XTICKUNITS = ['Time'], $
> XTICKFORMAT='LABEL_DATE', XSTYLE=1, XTICKS=6, YMARGIN=[6,2]
>
>
> end
>
>
>
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| Re: IDL 5.4 label_date function [message #27797 is a reply to message #27795] |
Wed, 07 November 2001 12:45 |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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From: "Craig Markwardt" <craigmnet@cow.physics.wisc.edu>
>
> "Chad Bahrmann" <cbahrmann@ou.edu> writes:
>
>> Has anyone encountered problems in using the label_date function in IDL
5.4?
>> From the documentation for IDL 5.4. This plot gives you data from
>> 11/01/2001 at 1200 through 11/02/2001 at 1200 when I would expect data
from
>> 11/01/2001 at 0000 through 11/02/2001 at 0000. I have had success using
>> this function in older versions of IDL but this one appears not to work
>> properly? The actual code had revisions on
> ...
>> ;Generate the Date/Time data
>> time = TIMEGEN(START=JULDAY(11,1,2001), FINAL=JULDAY(11,2,2001))
>
> I agree with David. This has nothing to do with LABEL_DATE, and
> everything to do with the Julian dates you created in your array.
> Midnight comes at Julian day fraction 0.5.
To expand on Craig's answer, Chad has been bitten by one of the curious
aspects of JULDAY. When it is given only three arguments (year, day, month)
it returns an integral Julian Date, representing 12:00 on the day in
question. When it is given another 3 (...hour,minute,second) it returns a
double Julian date, eg:
IDL> print, JULDAY(11,1,2001), JULDAY(11,1,2001,0,0,0)
2452215 2452214.5
This would have made sense 2000 years ago when the date changed at midday,
but these days it only makes sense to IDL developers.
And don't even start me on the perversity of the (year, day, month) order!
---
Mark Hadfield
m.hadfield@niwa.cri.nz http://katipo.niwa.cri.nz/~hadfield
National Institute for Water and Atmospheric Research
--
Posted from clam.niwa.cri.nz [202.36.29.1]
via Mailgate.ORG Server - http://www.Mailgate.ORG
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| Re: IDL 5.4 label_date function [message #27802 is a reply to message #27797] |
Wed, 07 November 2001 10:57 |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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"Chad Bahrmann" <cbahrmann@ou.edu> writes:
> Has anyone encountered problems in using the label_date function in IDL 5.4?
> From the documentation for IDL 5.4. This plot gives you data from
> 11/01/2001 at 1200 through 11/02/2001 at 1200 when I would expect data from
> 11/01/2001 at 0000 through 11/02/2001 at 0000. I have had success using
> this function in older versions of IDL but this one appears not to work
> properly? The actual code had revisions on
...
> ;Generate the Date/Time data
> time = TIMEGEN(START=JULDAY(11,1,2001), FINAL=JULDAY(11,2,2001))
I agree with David. This has nothing to do with LABEL_DATE, and
everything to do with the Julian dates you�created in your array.
Midnight comes at Julian day fraction 0.5.
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
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| Re: IDL 5.4 label_date function [message #27810 is a reply to message #27802] |
Wed, 07 November 2001 07:51 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Chad Bahrmann (cbahrmann@ou.edu) writes:
> Has anyone encountered problems in using the label_date function in IDL 5.4?
> From the documentation for IDL 5.4. This plot gives you data from
> 11/01/2001 at 1200 through 11/02/2001 at 1200 when I would expect data from
> 11/01/2001 at 0000 through 11/02/2001 at 0000. I have had success using
> this function in older versions of IDL but this one appears not to work
> properly? The actual code had revisions on
> ; MODIFICATION HISTORY:
> ; DMS, RSI. April, 1993. Written.
> ; DMS, RSI. March, 1997. Added Time format.
> ; DMS, RSI. Jan, 1999. Rounded least significant time unit
> ; CT, RSI. May 2000. Completely rewrote to use calendar format codes.
> ; Added Level argument for new date/time TICKUNITS.
> ; Added AM_PM and DAYS_OF_WEEK keywords, '%A' and '%W' codes.
> ;-
>
> PRO test
>
> ; Create format strings for a two-level axis:
>
> dummy = LABEL_DATE(DATE_FORMAT=['%H%I!C%N/%D/%Y'])
>
> ;Generate the Date/Time data
> time = TIMEGEN(START=JULDAY(11,1,2001), FINAL=JULDAY(11,2,2001))
>
> ;Generate the Y-axis data
> data = RANDOMN(seed, N_ELEMENTS(time))
>
> ;Plot the data
> PLOT, time, data, XTICKUNITS = ['Time'], $
> XTICKFORMAT='LABEL_DATE', XSTYLE=1, XTICKS=6, YMARGIN=[6,2]
>
>
> end
I don't know. The second paragraph in the TIMEGEN
documentation seems pretty clear:
The Julian date is the number of days elapsed
since Jan. 1, 4713 B.C.E., plus the time expressed
as a day fraction. Following the astronomical
convention, the day is defined to start at 12 PM
(noon). Julian date 0.0d is therefore Jan. 1,
4713 B.C.E. at 12:00:00.
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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