ivoa: DM type system

[Gerard Lemson]

Hi Arnold





On Tue, Apr 18, 2017 at 11:33 AM, Arnold Rots <arots at cfa.harvard.edu> wrote:

> Oh, and proleptic Gregorian. See section 3 of the WCS paper.
>
> ------------------------------------------------------------
> -------------------------------------------------
> Arnold H. Rots                                          Chandra X-ray
> Science Center
> Smithsonian Astrophysical Observatory                   tel:  +1 617 496
> 7701 <(617)%20496-7701>
> 60 Garden Street, MS 67                                      fax:  +1 617
> 495 7356 <(617)%20495-7356>
> Cambridge, MA 02138
> arots at cfa.harvard.edu
> USA
> http://hea-www.harvard.edu/~arots/
> ------------------------------------------------------------
> --------------------------------------------------
>
>
> On Tue, Apr 18, 2017 at 11:27 AM, Arnold Rots <arots at cfa.harvard.edu>
> wrote:
>
>> The datetime string (subset of ISO-8601, as defined in FITS WCS paper VI)
>> is specified as:
>>
>> [+|-C]CCYY-MM-DD[Thh:mm:ss[.s...]]
>>
>> 'T' is a literal, the other letters stand for single digits with obvious
>> meaning.
>>
>>   - Arnold
>>
>>
>>
>>
This still seems to be a definition in terms of a serialization that in the
VO-DML base types I would like to avoid.
Just like we do not want to be bothered by how an ivoa:real is represented
(ASCII-decimal, IEEE standard floating point, whatever).

I don't know what the best description of this serialization-independent
concept would be.
Would something like "the 0 component of some observer's four vector
identifying an event in 4-d space-time" be correct?

Cheers
Gerard



> ------------------------------------------------------------
>> -------------------------------------------------
>> Arnold H. Rots                                          Chandra X-ray
>> Science Center
>> Smithsonian Astrophysical Observatory                   tel:  +1 617 496
>> 7701 <(617)%20496-7701>
>> 60 Garden Street, MS 67                                      fax:  +1
>> 617 495 7356 <(617)%20495-7356>
>> Cambridge, MA 02138
>> arots at cfa.harvard.edu
>> USA
>> http://hea-www.harvard.edu/~arots/
>> ------------------------------------------------------------
>> --------------------------------------------------
>>
>>
>> On Tue, Apr 18, 2017 at 11:07 AM, Gerard Lemson <gerard.lemson at gmail.com>
>> wrote:
>>
>>> 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
>>>>
>>>>
>>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.ivoa.net/pipermail/dm/attachments/20170418/f46b5f2c/attachment.html>