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Actionfromtopurposestatus / answer

Check here for recent discussions:

Coordinates - 
The above page includes a proposal to add pos.bodycentric & pos.bodygraphic (which are not standard terms) to cover both the planets and the Sun (but not the Earth) 

With description:

  • Discussion : In body-centric coordinates, longitudes are defined from sub-observer point, while body-graphic coordinates are defined from a standard body reference frame

and a comment:
TODO: VESPA team to check consistency between heliocentric and planetocentric definitions 

First, the discussion is inadequate and instead relates to rotating frames. This has to be corrected along these lines (from IAU WGCCRE 2015 report)

-centric= a right-hand spherical coordinate system in which latitude is defined as the angle between a vector passing through the origin of the spherical coordinate system and the equator, and longitude is the angle between the vector projected onto the X Y plane and the positive X axis (the projection of the prime meridian on the X Y plan) measured in an eastern direction.
-graphic= The planetographic latitude of a point on the reference surface is the angle between the equatorial plane and the normal to the reference surface at the point. W longitudes (measured positively to the West) are used when the rotation is direct and E longitudes are used when the rotation is retrograde.
(we may want to simplify this a bit…)
 + notice that this only applies to planets and satellites (not to small bodies, and other conventions are mentioned for the Sun and Earth). 

Second, enlarging the concept to the Sun surface is certainly useful, although the term “heliocentric" is not used in WGCCRE reports and traditionally refers to something different (just like “geocentric“). 
Instead, these systems are defined in Thompson A&A 2006 - which states that “there is no distinction between planetographic and planetocentric latitudes and longitudes for the Sun“ and recommends to use the standard term “heliographic“ in this context.
So… shouldn’t we keep the widely used and unambiguous pos.planetocentric / pos.planetographic, and add pos.heliographic to these?

Use cases:

  • UCD to be associated with coordinates, e.g. in a VOtable, for use in tools. 
  • Can also be used for c1min, etc with body fixed frames in EPN-TAP:;pos.planetocentric (this is the reason to have a global term I guess).

But in both cases our spatial_coordinate_description parameter is more detailed (to be finalized)


IVOA semantics
(sent Oct 2019)

• Add date of query in return VOtable from Dachs (or ask how to add it)SEMarkusQueried to ~identify version of evolving services
• In HiPS metadata: hips_frame should accommodate planetary coordinate systems (when ready) in addition to celestial onesSEPierre FerniqueIdentify body and reference  frame
• Clarify conditions to reuse HiPS in external sotware (issue with CNES and possibly other environments)SEPierre Fernique


Actionfromtopurposestatus / answer

Datalink usage?

  • Question arises from duplicate granules providing alternative formats in EPN-TAP services. EPN-TAP v2 requires one granule / format in use (e.g. OMEGA_cubes service with both IDL and netcdf formats). Associated granules share the same obs_id (and other parameters) and have different access_format, so that different formats can be sorted out by a TAP query. This is nonetheless considered an issue for Aladin, and datalink is suggested to solve this.
  • However, grabbing a given entry in a datalink table associated to a granule is not easy in general. An alternative format may be provided through, e.g.:
    The point is: how do a portal or tool will grab files from this table for 20 selected granules?
  • First, there does not seem to be an automated procedure to retrieve a bunch of files from a datalink table in TOCAT or Aladin at the moment (TBC).
  • The current semantic vocabulary is too limited to identify an alternative format. This may improve in the short term with the addition of "sibling" or whatever new name (but remember than other types of files can be listed in this datalink table, such as raw data files…)
  • However, access would still require to pick-up a given mime-type from this table. 

Altogether, the use/benefit of datalink to improve access to alternative file formats is not very clear: this requires a (new?) mechanism in tools; a service-specific tweak; and a multiparameter query.

SE, discussion with PFernique