Glad to see this subject poping up, Francois Ochsenbein and I had a small
exchange of ideas a few weeks back.
Mark's explanation is quite good, but I would like to add a couple of things:
a "datetime" (however it is represented) corresponds to an instant in time.
Julian Date and Modified Julian Date (which do assume the usage of UTC) seem to be one of the most appropriate way of representing such instant in time. Using the ISO standard (string representation) is quite common, but any application wanting to compare instants in time needs to convert it to a floatinng number (double).
Instants in time may mean a number of different things, which at this point we don't have any way to handle (more later).
One of the problems I'm working on at the moment deal with comparing "simultaneity" or overlapping of time intervals. In this case, the application requires two instants in time: begining of event and end of event.
One of the sample VOTables I'm using has five (yes 5) times listed, and as it is an observing log, they represent
1- begining of observation 2- end of observation 3- begining of event (a solar flare) 4- end of event 5- mid point of observation.
UCDs don't have the structure to let us differentiate one "instant" from another, units don't help either, utype? Perhaps, but few people are using it yet, column explanation? We're lucky if there is one attached!
At the end, the application I wrote let the user specify the columns for start_event & end_event...
And those 5 examples are not the only ones, we could have
Seems to me that we need to: first agree in valid/meaningful ways of representing time, and spread the voice urbe et orbi, as we'll soon start wanting to know which events were observed with given instruments, in particular if the logs are available in the VO.
second, start thinking how to identify the different things that an instant of time may mean, and to start, encourage people generating data to add meaningful explanations to the columns, so if the user is forced to make a decision by hand, at least it is an informed one. The sample VOTables I'm using right now don't have such explanations attached automagically yet I'm sure that information is available somewhere.
Cheers,
Patricio
On Fri, 22 Apr 2005, Mark Taylor wrote:
> On Fri, 22 Apr 2005, Tony Linde wrote:
>
> > Can I ask why datetime is not a datatype in VOTable?
> >
> > Cheers,
> > Tony.
>
> Because there is no corresponding FITS BINTABLE TFORMn value; see:
>
> http://www.ivoa.net/Documents/REC/VOTable/VOTable-20040811.html#ToC11
> http://archive.stsci.edu/fits/fits_standard/node68.html#3124
>
> VOTable takes its idea of what datatype a FIELD (or PARAM) can store
> from the FITS binary table specification and not from XML Schema
> (or anywhere else) and thus it's pretty low-level - there isn't even a
> String type as such, you can only represent one as an array of characters.
> The only type in VOTable but not FITS is unicodeChar, which is
> presumably allowed because there is a clear 1:1 mapping between
> UCS-2 Unicode characters and short integers.
>
> The datatype attribute is there to tell VOTable parsers how to
> turn the content of a TD element or some bytes in a BINARY (and
> possibly FITS) stream into a numeric/character value. Any semantic
> interpretations put on those numbers/characters is up to the
> application, perhaps by looking at other attributes such as
> ucd/utype/units. The most obvious place to note that the
> content of a particular column is intended to be understood as
> a datetime-type value would probably be the units attribute, but
> I don't think there is any standard way of doing this.
>
> It would certainly be useful for generic VOTable-consuming applications
> if there was a standard way of telling that a string should be
> interpreted to mean a date (or sky coordinate, ...) in a particular way,
> but at the moment the VOTable standard does not operate at this level.
>
> Mark
>
> --
> Mark Taylor Starlink Programmer Physics, Bristol University, UK
> m.b.taylor@bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/
>
>
--- Patricio F. Ortiz pfo-at-star.le.ac.uk Department of Physics & Astronomy Phone: +44 (0)116 252 2015 University of Leicester Leicester, LE1 7RH, UKReceived on 2005-04-22Z19:27:05