POL (Re: About the draft itself Re: WD-SIA-2.0)

[Arnold Rots]

In STC2.0 I am recognizing four types of polarization coordinates, all
enumerated:

POLSTOKES: I, Q, U, V
POLCIRC: RR, LL, RL, LR
POLLIN: XX, YY, XY, YX
POLVEC: I, PF, PP, PA

   - Arnold

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Tue, May 6, 2014 at 12:26 PM, Patrick Dowler <
patrick.dowler at nrc-cnrc.gc.ca> wrote:

>
> On 14/04/14 01:00 PM, Arnold Rots wrote:
>
>> Since polarization is an enumerated axis, I think POL=I,Q,U,V should be
>> perfectly acceptable, but POL=I/V is not.
>> Note that by explicitly endorsing Stokes parameters, other polarization
>> expressions (circular, linear, vector) are excluded.
>>
>
> Querying the polarization axis for (ObsCore) pol_states that are present
> in the data is not straightforward.
>
> Using repeated parameters (as is done when you pick multiple things from a
> picklist), eg:
>
> POL=I
> POL=Q
> POL=U
>
> would generally mean that the data has to contain at least one of these.
> Changing that behaviour is wide-reaching and DAL has long ago (Nara, iirc)
> decided to use this in place of comma-separated lists for multiple input
> values. Using a range construct, eg:
>
> POL=I/U
>
> would mean the same thing and find the same data (in this particular
> example but not for all). In a TAP+ADQL query of the ObsCore table, one
> could look for exactly IQUV with
>
> pol_states like '/I/Q/U/V/%'
>
> as described in ObsCore-1.0 B.6.6 (no wildcard at front because I is first
> in the canonical ordering). That would do what people want, leading me to
> think that
>
> POL=I Q U V
>
> (suitably separated and/or encoded) is the right way to say "search for
> data with all of these states".
>
> Now the mess: ObsCore-1.0 says that the separator between pol_state values
> is the slash (/) character and range querying in DAL also uses the /
> character. The first was essentially an adhoc decision; the second comes
> from previous use in DAL services and is based on some external param-based
> query spec (can't recall, was in geoscience, iirc) where / was used because
> it is straightforward to split  and doesn't interfere with scientific
> notation, ISO8601 timestamp values, etc.
>
> Technically, the ADQL constraint would be: pol_states like '/I/Q/U/V%' and
> the POL parameter cannot use that string.
>
> So, is searching for exactly I Q U V easily supported in the POL
> parameter? If in a single parameter, what separator? Is that a general
> pattern for other enumerated fields (personally: I doubt it: I have not run
> into another enumerated field with multiple values like pol_states)
>
>
> PS - In my opinion, any special treatment means we are in for some complex
> language and a single very peculiar parameter if we try to support this. I
> think we should stick with multiple values and ranges and if someone needs
> more explicit query capability they can/should use TAP, where the hard and
> general problems are dealt with.
>
> --
>
> Patrick Dowler
> Canadian Astronomy Data Centre
> National Research Council Canada
> 5071 West Saanich Road
> Victoria, BC V9E 2E7
>
> 250-363-0044 (office) 250-363-0045 (fax)
>
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