ivoa: DM type system

[Gerard Lemson]

Hi Mark

On Mon, Apr 17, 2017 at 11:51 AM, CresitelloDittmar, Mark <
mdittmar at cfa.harvard.edu> wrote:

>
>
> ...
>>
> I don't think this needs to be complicated, so let's put these on the back
> burner.
>
> Thanks!

>
> >>     * Quantity and datetime would not have a common ancestor, so we
> could not define a TimeStamp
> >>       which would allow time represented as a RealQuantity OR datetime.
> >>       https://volute.g-vo.org/svn/trunk/projects/dm/STC-
> <https://volute.g-vo.org/svn/trunk/projects/dm/STC-2.0/doc/diagrams/alt/temporal%20domain%20diagram.png>
> 2.0/doc/diagrams/alt/temporal%20domain%20diagram.png
> <https://volute.g-vo.org/svn/trunk/projects/dm/STC-2.0/doc/diagrams/alt/temporal%20domain%20diagram.png>
> >>
>
> >So are you saying you'd like a DatetimeQuantity? With only a "datetime
> unit" being sufficient?
> >I had hoped   'ivoa:datetime' would be sufficient and the precise
> representation would be left to the mapping.
>
> This sounds like you are suggesting that 'ivoa:datetime' should suffice
> for all uses.
>    MJDREF  = 50814.02               / [d] zero point for times - MJD
>    TSTART  = 84244214.7546979934 <(754)%20697-9934>    / [s] Observation
> start time
>    DATE-OBS= "2000-09-02T01:10:14"  / Date and time of observation start
>
> I want all of these to be a TimeStamp type (abstract).
>   * datetime = has vodml/html description:  "Represents a moment in time
> using a date+timestamp."
> which covers 'DATE-OBS', the other 2 are satisfied by RealQuantity.
>
> But.. since RealQuantity and datetime do not have a common ancestor, I
> cannot define TimeStamp without either an anyType or a DateQuantity.
>
>
Yes, that is correct, but I see now (I think) what might cause confusion,
namely the description of  ivoa:datetime in the model as
"Represents a moment in time using a date+timestamp". What was meant there
is that it is not just a day/date (2017-04-18), and not just a time
("13:00:01.3"), but the fully specified moment in time.
In the current description it does indeed sound as if date and time must be
represented, i.e. too much like a prescription for valid serializations.
This was NOT intended.
ivoa:datetime is supposed to represent the concept of a Timestamp as you
mention it, and it should be up to serialization prescriptions to take care
of valid representations.

So if that interpretation is given to ivoa:datetime, would that type be
sufficient for your requirements?

Cheers
Gerard




> Mark
>
>
>
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