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Date: Mon, 9 Feb 1998 13:23:33 -0500 (EST)
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From: Don Wells <dwells at nrao.edu>
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Subject: A warning for FITS y2k implementors
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An implementor of FITS y2k code told me:

  "..there was one sort of amusing problem with the initial
  implementation that other systems might run into. By default with the
  new-style we include the time. But because of floating point rounding,
  the new style date would turn into previous_day:T23:59:59.99 or
  whatever. While this is fine for systems that read the time part of
  the new format, it would presumably cause systems that ignore the time
  part to think the date was a day earlier than it actually was.."

-Don [Chair, IAU FITS Working Group]
-- 
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
                    http://fits.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA


From owner-fitsbits  Tue Feb 10 10:42:46 1998
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Date: Tue, 10 Feb 1998 15:40:03 +0000
From: Peter Bunclark <psb at ast.cam.ac.uk>
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Subject: Re: A warning for FITS y2k implementors
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Looks suspiciously like truncation rather than rounding, hence
is a bug not a feature...  and of course, if you choose to
read the date but not the time you must surely expect up to 24-hour
quantisation errors?  It's a bit like reading a floating-point value
up to the decimal point and ignoring the rest!

Peter.

Don Wells wrote:
> 
> An implementor of FITS y2k code told me:
> 
>   "..there was one sort of amusing problem with the initial
>   implementation that other systems might run into. By default with the
>   new-style we include the time. But because of floating point rounding,
>   the new style date would turn into previous_day:T23:59:59.99 or
>   whatever. While this is fine for systems that read the time part of
>   the new format, it would presumably cause systems that ignore the time
>   part to think the date was a day earlier than it actually was.."
> 
> -Don [Chair, IAU FITS Working Group]
> --
>   Donald C. Wells         Associate Scientist         dwells at nrao.edu
>                     http://fits.cv.nrao.edu/~dwells
>   National Radio Astronomy Observatory                +1-804-296-0277
>   520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA


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To: fitsbits at fits.cv.nrao.edu
Date: 10 Feb 1998 18:48:43 GMT
From: thompson at orpheus.nascom.nasa.gov (William Thompson)
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References: <199802091823.NAA08899 at fits.cv.nrao.edu>
Subject: Re: A warning for FITS y2k implementors
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Don Wells <dwells at NRAO.EDU> writes:

>An implementor of FITS y2k code told me:

>  "..there was one sort of amusing problem with the initial
>  implementation that other systems might run into. By default with the
>  new-style we include the time. But because of floating point rounding,
>  the new style date would turn into previous_day:T23:59:59.99 or
>  whatever. While this is fine for systems that read the time part of
>  the new format, it would presumably cause systems that ignore the time
>  part to think the date was a day earlier than it actually was.."

Something else that implementers of the new standard should be aware of is that
under certain circumstances, a time such as 

	1997-06-30T23:59:60.999

could also be a valid time string, on dates containing leap seconds.

William Thompson


From owner-fitsbits  Tue Feb 10 16:44:52 1998
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To: fitsbits at fits.cv.nrao.edu
Date: 10 Feb 1998 21:33:27 GMT
From: seaman at noao.edu (Rob Seaman)
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Subject: Re: A warning for FITS y2k implementors
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Peter Bunclark <psb at ast.cam.ac.uk> writes:

> Looks suspiciously like truncation rather than rounding, hence
> is a bug not a feature...  and of course, if you choose to
> read the date but not the time you must surely expect up to 24-hour
> quantisation errors?  It's a bit like reading a floating-point value
> up to the decimal point and ignoring the rest!

Bundling the date and time into the same keyword is also no different in
this regard than having separate DATE-OBS and UT (for instance) keywords.
Code that previously generated correct UT information should still work
when grafted onto the DATE-OBS keyword.

If the problem is that this implementor wrote some generic routine that
just divided out all of the date and time fields from one unsegmented
time argument (seconds since 1970 or whatever), and that only arguments
corresponding to even date numbers were being fed to the routine for
some reason, resulting in the 23:59:59.9 vs. 00:00:00.0 chatter around
midnight - well, this seems like an inappropriate usage.

Note that there were various discussions regarding "atomic" date/time
pairs when we were thrashing out the MJD and Y2K stuff.  Maintaining
an unsegmented time scale is a larger issue than the ISO format, and
certainly than rounding or truncation errors.

DATE-OBS specified as a variation of YYYY-MM-DDThh:mm:ss[.sss...] consists
of seven separate time segments that have to be reliably generated from
some unsegmented clock.  The fractional seconds field shouldn't just be
calculated, but should have some reality checks built into the logic.

Obviously, the simpler sexigesimal carrying and the more complicated year-
and-month dependent calendar issues have to be handled cleanly or the whole
Y2K bug will recur in some other guise.  Also, whatever TIMESYS is being
used for the date should also be used for the time information, and the
calendar and clock should be consulted simultaneously (not a trivial
computer science issue).

I would also hazard that if some software cares about the date, but not
about the time (i.e., only cares about the time to Peter's 24 hour
granularity), that a careful programmer would parse the time as well as
the date, and would round to the nearest day.  That FITS may now require
some logic tree to decide whether the time comes from DATE-OBS or UT or
whatever other keyword is just an added expense of doing FITS now that
we've added this feature of DATE-OBS.

Rob
-- 
seaman at noao.edu, http://iraf.noao.edu/~seaman
NOAO, 950 N Cherry Ave, Tucson AZ 85719, 520-318-8248
PGP: 98 8D 8B 49 74 9A 41 88  3A 43 87 54 51 BF 30 4B


From owner-fitsbits  Wed Feb 11 09:34:22 1998
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To: fitsbits at fits.cv.nrao.edu
Date: 10 Feb 1998 15:20:08 -0800
From: sla at ucolick.borg (Steve Allen)
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Subject: Re: A warning for FITS y2k implementors
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In article <6bq7eb$l0o at post.gsfc.nasa.gov>,
William Thompson <thompson at orpheus.nascom.nasa.gov> wrote:
>under certain circumstances, a time such as
>
>       1997-06-30T23:59:60.999
>
>could also be a valid time string, on dates containing leap seconds.

There are few, if any, computer operating systems which implement a
correct model of time.

Therefore extreme caution should be used if new FITS date strings are
to be converted to operating system clock times using standard system
library procedures.
-- 
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.borg     Voice: +1 408 459 3046     FAX (don't): +1 408 454 9863
WWW: http://www.ucolick.borg/~sla               PGP public keys:  see WWW
Junk mail is irrelevant -- my return address has been assimilated.


From owner-fitsbits  Wed Feb 11 09:37:01 1998
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To: fitsbits at fits.cv.nrao.edu
Date: Tue, 10 Feb 1998 23:53:20 -0600
From: Ray Plante <rplante at ncsa.uiuc.edu>
Message-ID: <34E13CD0.3D8C2B2D at ncsa.uiuc.edu>
Organization: National Center for Supercomputing Applications
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Subject: FITS & the Horizon Java Package
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Dear Colleagues,

NCSA and Project 30 is pleased to announce the latest release of the
Horizon Image Data Browser, version 1.3 beta, a Java package for
browsing scientific images.  This package may be downloaded from the
Horizon IDB Home Page: 

    http://imagelib.ncsa.uiuc.edu/Horizon

Included with the latest version is a demonstration version of a FITS
Image Browser.  Here are some of its features:

    * Basic 2-D visualization of FITS images from either local disk 
      or over the network.
    * Zooming and panning
    * Color map and transfer function control over a selectable data
      range
    * 2-D slice selection for mult-dimensional datasets
    * data value and world coordinate position displays, including
      built-in support for projected spherical systems commonly used
      in astronomy and earth science.  

You can try this viewer out from our Web page, but you can also download
the Horizon package and run it as a standalone application (with
improved performance).  

This latest version, 1.3 beta, represents the last major release that is
compatible with Java 1.0.2.  The next major version, 2.0 beta, will be
Java 1.1 compatible.  (Some minor versions may be released for bug
fixes.)  Some of the new features slated for the next version of the
FITS Image Browser includes spreadsheet display, metadata display, color
table editing, as well as support for collaboration.

We encourage you to download the Horizon package and try it out.  We
also welcome your comments, questions, and suggestions for
improvements.  If you would like to be added to our mailing list for
notices of new releases, just send me email at horizon at ncsa.uiuc.edu.

have fun,
Ray Plante
NCSA Horizon Development Team


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From: pence at tetra.gsfc.nasa.gov (William Pence)
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To: fitsbits at fits.cv.nrao.edu
Subject: New CFITSIO Release
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                          CFITSIO 1.40 Release
                            11 February 1998

A new version of the CFITSIO library for reading and writing FITS
format data files is now available.  This release contains several
significant new features that affect both C and Fortran users of the
CFITSIO and FITSIO libraries:


New For C Programmers:
______________________

CFITSIO can now directly read compressed FITS files in gzip (.gz) or
Unix compress (.Z) formats.  Users can save time and disk space by
reading the compressed FITS file instead of having to uncompress it
first.

FITS files can be read from the 'stdin' file stream and written out to
the 'stdout' stream.  Just specify the input or output FITS file name
as "-" (a minus sign) to use this feature.  This enables a sequence of
programs to pass FITS files from one to the other in memory thus
speeding up the processing compared with having to create intermediate
FITS files on disk.

This release introduces a powerful new function called the 'iterator'
for reading and writing data in FITS images or tables.  This function
can simplify the way software reads and writes images or columns of
data, and make the code more 'object oriented' by cleanly separating
the data I/O from the algorithms that operate on the data.  This
routine can greatly reduce the number of lines of code that are needed
to read or write the data and can improve the efficiency of the program
at the same time.  Several sample programs that use this iterator
function are included in this release.

The User's Guide has been restructured to make it easier and faster to
learn to use CFITIO.  The most commonly used 'basic' interface routines
have been grouped into one chapter for easy reference.  Several new
high-level routines have also been added to simplify the interface by
combining what used to require several function calls into one call.


New for Fortran Programmers:
___________________________

CFITSIO (written in ANSI C) now includes a set of Fortran-callable
wrapper routines which effectively replace the old FITSIO Fortran
library.  These new CFITSIO wrappers have exactly the same calling
sequence as the old FITSIO routines, so any Fortran program that
currently uses the FITSIO library can now be linked instead to the
CFITSIO library without any change to the source code.  The main
advantage of using these new CFITSIO wrappers instead of FITSIO are:

  - they support the new features in CFITSIO (like reading compressed
    FITS files, and the new iterator function) that are not available
    in FITSIO.

  - they are generally faster, by a factor of 2 in some cases.

These C wrappers work on most Unix platforms, including linux, and on
VMS systems.  They currently are not supported on Cray supercomputers
and on Windows PCs and Macs.

Fortran programmers are urged to start using the CFITSIO library
because support for FITSIO will now be limited to simple bug fixes and
will not include any new features added to CFITSIO.


Where to get it:
________________

CFITSIO is available on the web or by anonymous ftp from: 


   http://heasarc.gsfc.nasa.gov/fitsio
   ftp://legacy.gsfc.nasa.gov/software/fitsio/c

_________________________________________________________________________ 
William Pence
HEASARC - High Energy Astrophysics Science Archive Research Center
NASA/GSFC
e-mail: pence at tetra.gsfc.nasa.gov
phone: (301)286-4599