VEP-017: refframe#geo_app

Wed Nov 20 21:47:13 CET 2024

Hi all,

We did some digging with Baptiste and Mireille before ADASS / Interop on 
this geo_app
term, and I take this VEP as an opportunity to report on what we found.

TL/DR : geo_app most likely corresponds to the GAPPT FITS keyword, for
the geocentric apparent equatorial coordinate system.
And VEP-017 looks good !

Detailed search (for those who might dig into this 25 years from now) :

- geo_app has been a possible value of COOSYS in VOTable IVOA Rec since 
the beginning:
https://www.ivoa.net/documents/VOTable/20031017/PR-VOTable-1.0-20031017.html
and even before IVOA, in the first draft (v0.1 18-Nov-2001 ) :
https://cds.unistra.fr/doc-cds/VOTable/

- geo_app was also present as a possible value for coosys element in XML 
precursors of
VOTable, namely :
    - astrores (discussions from ADASS 1999 to January 2001) :
https://vizier.cds.unistra.fr/vizier/doc/astrores.htx
    - AML (Astronomical Markup Language), in Damien Guillaume's PhD, p. 
112 (Jan. 2000) :
https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7b45eb756c6887b9cdb967998369f44c7a660024

(Internet Archive Wayback machine if some links disappear :
https://web.archive.org/web/20000526191744/http://vizier.u-strasbg.fr/doc/astrores.htx 
)

- The exact origin of coosys and its allowed values are hard to trace, there
are few relevant links :
https://web.archive.org/web/19990901020112/http://pioneer.gsfc.nasa.gov/xml/internal/mail/mhonarc/xml/msg00005.html
http://lists.xml.org/archives/xml-dev/

- Most likely, geo_app corresponds to the *GAPPT* FITS keyword in Table 2 of
Calabretta & Greisen 2002 "Representations of celestial coordinates in 
FITS" :
https://www.aanda.org/articles/aa/pdf/2002/45/aah3860.pdf

> GAPPT : geocentric apparent place, IAU 1984 system
>
> Geocentric apparent equatorial and ecliptic coordinates
> (GAPPT) require the epoch of the equator and equinox of date.
> This will be taken as the time of observation rather than
> EQUINOXa.

- It is also described in the documentation of the Starlink AST WCS library
https://starlink.eao.hawaii.edu/docs/sun211.htx/sun211ss454.html
> |"| GAPPT|"| , |"| GEOCENTRIC|"| or |"| APPARENT|"| : The geocentric 
> apparent equatorial coordinate system, which gives the apparent 
> positions of sources relative to the true plane of the Earth|’| s 
> equator and the equinox (the coordinate origin) at a time specified by 
> the qualifying Epoch value. (Note that no Equinox is needed to qualify 
> this coordinate system because no model |"| mean equinox|"| is 
> involved.) These coordinates give the apparent right ascension and 
> declination of a source for a specified date of observation, and 
> therefore form an approximate basis for pointing a telescope. Note, 
> however, that they are applicable to a fictitious observer at the 
> Earth|’| s centre, and therefore ignore such effects as atmospheric 
> refraction and the (normally much smaller) aberration of light due to 
> the rotational velocity of the Earth|’| s surface. Geocentric apparent 
> coordinates are derived from the standard FK5 (J2000.0) barycentric 
> coordinates by taking account of the gravitational deflection of light 
> by the Sun (usually small), the aberration of light caused by the 
> motion of the Earth|’| s centre with respect to the barycentre 
> (larger), and the precession and nutation of the Earth|’| s spin axis 
> (normally larger still). 

Cheers,
Sébastien

Le 20/11/2024 à 15:02, Markus Demleitner via dm a écrit :
> Dear Semantics, Apps, and DM WGs,
>
> In our refframe vocabulary, we have a term geo_app that we have
> inherited from VOTable's COOSYS/@system.  When we created the
> vocabulary, nobody could quite remember what it was supposed to mean,
> in particular because the "geo" part made us suspect it would have
> been a frame for Earth.
>
> At the recent Interop, JJ Kavelaars recognised what this was supposed
> to mean: geocentric apparent positions.  Even though the term is
> deprecated (and we will not un-deprecate it for reasons mentioned
> below), we should fix the ugly label and description.  This is what
> VEP-017 tries:
>
>    Vocabulary:http://www.ivoa.net/rdf/refframe
>    Author: Markus Demleitner<msdemlei at ari.uni-heidelberg.de>  and J.J. Kavelaars
>    Date: 2024-11-17
>
>    New Term: geo_app
>    Action: Modification
>    Label: Geocentric Apparent Positions
>    Description: Positions given as observed by a fictitious observer at the
> 	  Earth’ s centre for the equator of observation.
>    Relationships: narrower than #EQUATORIAL
>    Used-in: VOTable legacy
>
>    Rationale:
>    When the legacy VOTable terms were added to the refframe vocabulary
>    (which was necessary to keep validity criteria when VOTable changed
>    COOSYS/@system to being controlled by this vocabulary), nobody rememberd
>    what geo_app might have meant.  It recently turned out that it must
>    have been “Geocentric Apparent”, a frame used primarly in geodesy and
>    solar system science.  It is documented to some extent in
>    https://starlink.eao.hawaii.edu/docs/sun211.htx/sun211ss454.html.
>
>    This VEP replaces label, description, and relationship for the concept.
>    At this point, this is a deprecated concept, and since it is mixing
>    refposition and frame ("geocentre" vs. "apparent positions at equator of
>    epoch"), we quite certainly won't want anyone to use it.  We still want
>    to update the concept metadata and hence ask for exemption from the
>    “must be used somewhere” rule.
>
> https://github.com/ivoa-std/VEPs/blob/d79185c3a36bf704b84fe439ba4513b8cfb5304e/VEP-017.txt
>
> Comments?  Thoughts?  I'd say this is reasonably harmless, and so I'd
> suggest to bring this to the TCG in about two weeks if nobody
> protests here.
>
> Thanks,
>
>               Markus
>

-- 

    (((    Sebastien Derrieresebastien.derriere at astro.unistra.fr
   (. .)   Observatoire de Strasbourg       Phone +33 (0) 368 852 444
  (( v ))  11, rue de l'universite        Telefax +33 (0) 368 852 432
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