Inputs for UCDs

[Stéphane Erard]

Hello

In https://wiki.ivoa.net/twiki/bin/view/IVOA/UCDList_1-3_RFM
there is 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… there may be reasons to keep the widely used and unambiguous pos.planetocentric / pos.planetographic, and add pos.heliographic to these - still TBD.

Cheers,
Stéphane


> Le 9 oct. 2019 à 18:54, Stéphane Erard <stephane.erard at obspm.fr> a écrit :
> 
> Dear all
> 
> Here are some comments I’ve collected since the last Interop Meeting about UCDs, in relationship with Planetary Science / coming EPNCore standard. 
> Sorry for the short notice ;(
> 
> Cheers
> Stéphane
> 
> 
> Maps:
> • At least a new pos.projection is required to introduce projection/mapping schemes (there is currently only a very specific pos.lambert). 
> 
> VOevents:
> • UCDs are needed for VOevents properties
> 
> 
> For reflected light measurements:
> 
> • Correct/clarify definition of reflectance in 2018 RFM sheet: 
> add Q | phys.reflectance | Radiance factor (received radiance divided by input radiance)
> => Radiance factor is the same as I/F; in this case, this is divided by input irradiance (not radiance), i.e. by the solar flux.
> 
> • Correct/clarify brdf in 2018 sheet: 
> addQ | phys.reflectance.bidirectional.df| Bidirectional reflectance distribution function
> 
> This UCD is very weird, would better be phys.reflectance.brdf (this is not a subset of .bidirectional, and dfl means nothing in this context while brdf is a common acronym)
> definition: Ratio of radiance to incident normal solar flux
> 
> • Suggestion for reflectance in 2018 RFM sheet, to be corrected: 
> for normalized reflectance concept: proposal arith.ratio;phys.reflectance 
> definition : reflectance normalized per reflectance at one wavelength 
> => Should be arith.factor;phys.reflectance
> 
> 
> Other UCDs:
> • posAng is commonly mistaken for a generic UCD for angles (which does not exist), while it is specifically for position angles. 
> => Make definition more explicit (e.g., from Free dictionary: the direction in which one object lies relative to another on the celestial sphere, measured in degrees from north in an easterly direction)
> 
> • How do we handle hemispherically integrated quantities?
> 
> • arithm.ang could do as S to specify angular distributions
> 
> • Section 8, 2 (about pos section): 
> "the angular size of an object is in this section, its linear size is in the phys section)"
> But we have phys.angSize (with an unclear definition)  - should it be in pos then?
> 
> • On the same token, 
> phys.angVeloc would look more consistent than phys.veloc.ang
> 
> • How do we specify an additive constant or offset? 
> The only close match is arith.zp, which is context specific (for magnitude scales)
> => Enlarge the definition to any type of offset / additive constant?
> 
> • We have compiled a preliminary list of measured spectral quantities - get prepared for this ;)
> In particular, a category phys.scattering seems required, see here: 
> https://voparis-wiki.obspm.fr/display/VES/Lab+spectroscopy+extension
> (Table 2, column 3 for proposals)
> 
> 
> + I’m not sure if this is for here, but I also noted:
> • Clarify usage of inf / -inf / Nan in ADQL (should apparently exist, as per DALI - but do not)
> 
> 
> 
> 
>