UCD vocabulary revision : first iteration of the maintance procedure

[Stéphane Erard]

Hello 

Le 23 mai 2018 à 21:01, Mireille Louys <mireille.louys at unistra.fr> a écrit :

> Hi Stephane , Hi all, 
> My comments inserted below... 
> Le 23/05/2018 à 01:00, Stéphane Erard a écrit :
>> Dear Mireille and all
>> 
>> I think some elements of the original requests got lost in the process. 
>> 
> yes there are pending terms for which the decision to propose was not clear . They will be discussed in the next iteration.
>> Le 21 mai 2018 à 00:30, Mireille Louys <mireille.louys at unistra.fr>
>>  a écrit :
>> 
>> 
>>> Dear Semantics , dear tcg members 
>>> 
>>> The Semantics has proposed a Maintenance procedure in oder to update the UCD list ( and more generaly ivoa vocabularies) 
>>> as stated in the WD 
>>> http://www.ivoa.net/documents/UCDlistMaintenance/20171018/
>>> 
>>> 
>>> We are experiencing the first iteration of this procedure.
>>> 
>>> Here is the list of new terms asked during the last revision iteration for UCD terms asked for the period  "May 2017 - Feb2018"
>>>  23 terms have been proposed, discussed in the UCD expert group and inserted in the UCD assigning tool.
>>>  
>>>   Q | phys.electCharge | Electric charge 
>>>   Q | phys.current | Electric current
>>>   Q | phys.current.density | Electric current density
>>>   Q | pos.incidenceAng | Incidence angle of optical ray on an interface
>>>   Q | pos.emergenceAng | Emergence angle of optical ray on an interface
>>>   Q | pos.azimuth | azimuthal angle in a generic reference plane
>>> 
>> My original proposal was pos.azimuthAng 
>> Is there a reason to have a syntax different from the other angles? (pos.phaseAng which is related has been there for a long time) 
>> A more specific definition is
>> 	Angle between illumination and viewing directions projected on the horizontal plane
>> 
>> 
>>>   Q | phys.reflectance | Radiance factor (received radiance divided by input radiance)
>>> 
>> This description is not very precise: Radiance factor or I/F (radiance over normal solar flux)
>>  (to make it short…)
>> 
> I/F cannot be used in our definition label , because the UCD builder needs to allow a token for each term and weight them in order to define candidate 
> UCD terms for assignment. 
> The quantities to consider in this vocabulary should not depend (as far as possible) on any letter conventions.  
> "radiance over normal solar flux" would be appropriate terms for the assignation algorithm behind the UCD builder @CDS .  
> Do you consider this definition is  precise enough for your use-cases ? 

I/F can be written explicitly "I over F ratio" - this is standard for astronomers (those looking at planets, though…). "Reflectance factor" is another standard naming, more related to lab spectroscopy; they happen to designate the same quantity.

> 
>>>   Q | phys.reflectance.bidirectional | Bidirectional reflectance
>>>   Q | phys.reflectance.bidirectional.df | Bidirectional reflectance distribution function
>>> 
>> This one is a misunderstanding: "distribution function" is not a subtype of bidirectional reflectance (and has nothing to do with a probability distribution function). Instead, both bidirectional and brdf (= bidirectional reflectance distribution function) are subtypes of reflectance. brdf is a standard acronym in the interested community.
>> I suggest instead to have
>>  Q | phys.reflectance.bidirectional | Bidirectional reflectance (as proposed)
>>  Q | phys.reflectance.brdf | Bidirectional reflectance distribution function
>> If this is an issue, the first one has priority
>> 
> if you need the precise term ' BRDF' I suggest then to adopt your suggestion : 
>  Q | phys.reflectance.brdf | Bidirectional reflectance distribution function
> but 
> would you consider the meaning is correct with 
> arith.ratio; phys.reflectance.bidirectional ?

In this case it won't do: this is not the ratio of 2 similar quantities. 
In fact brdf =  phys.reflectance / pi / cos (incidence angle)
So this is the same physical quantity (unit = sr**-1 in SI), but another scaling.  
We have an issue with these quantities, in short : all spectral tools use UCD to identify physical dimensions and proper scaling, and since reflectances are dimensionless we can't include these differences in UCD with coefficients. 
Plus, the quantity which is more useful than brdf is actually Reflectance factor (no kidding…) = phys.reflectance / cos (incidence angle)
this is the normal scaling of laboratory data, and we already have several data services distributing such data.
I know this is a mess, but planetary spectroscopy really needs at least:

Q | phys.reflectance | Radiance factor or I over F ratio (received radiance divided by input radiance)
Q | phys.reflectance.reff | Reflectance factor (radiance factor divided by cos of incidence)
both are bidirectional and dimensionless, and are most useful for describing reduced data. 
The other 2 are the same divided by an extra factor of pi in practice (they relate to another estimate of the input flux)
If symmetry is important we can also use:
 phys.reflectance.radf | Radiance factor
 phys.reflectance.reff | Reflectance factor 
In which case phys.reflectance can be used as generic reflectance, with arbitrary scaling (ie, uncalibrated). This looks like the best solution in fact.
The acronyms radf and reff are recognized in this community.

Sorry about this… ;)
> 
> 
> I suggest we try to continue the discussion on this list. 
> Thank you all for your precious feedback . 
> --
> MireilleLouys , Semantics chair

Stéphane