[QUANTITY] moving on

[Jonathan McDowell]

Alberto wrote:

> I still think that UTYPE will not be limited to VOTable application.
> We need to be able to distinguish between measurements of the same
> thing (e.g. the diameter of a galaxy) which
> were taken using different recipes, and are therefore not directly
> comparable (see http://www.ivoa.net/forum/dm/0456.htm )
> It is the UTYPE that could make the trick to know whether two quantities
> are really compatible (same UTYPE), or could even give us the hook to a 
> repository of recipes which could be used to make the Mapping
> between two quantities (different -but similar- UTYPEs, same UCD).
> 
> Alberto
> 
> 


In your example, we are talking about the isophotal diameter
of a galaxy, and the issue is that this is a parameterized concept
- it depends on which isophote you choose, e.g. 25 mag/arcsec2 in V
or 23 mag/arcsec2 in B.

It seems to me that here we have three UCD/concepts:
 - isophotal diameter
 - frequency or bandpass
 - surface brightness
and one data model:
 isophotal diameter has a value, and has metadata of bandpass and 
  surface brightness (SB) contour.

In Martin's approach we would make an explicit IsophotalDiameter model
which knew how to link these three UCDs. 

<IsophotalDiameter ucd="pos.diam.isophotal" unit="arcmin">
 <Bandpass ucd="em.bandpass">V</Bandpass>
 <SurfaceBrightness ucd="flux.sb" unit="mag/arcsec^2">24</SurfaceBrightness>
 <Value>3.8</Value>
</IsophotalDiameter>


In the Quantity approach  I
propose we would store the bandpass and SB as metadata BasicQs within
the diameter Q, and rely on a generic rule to say that two Qs are
equivalent if they have the same UCD *and* if their metadata agree in
value. 

<Quantity name="IsophotalDiameter" ucd="Pos.diam.isophotal">
 <Quantity name="Bandpass" ucd="em.bandpass">V</Quantity>
 <Quantity name="SurfaceBrightness" ucd="flux.sb" unit="mag/arcsec^2">24</Quantity>
 <Value>3.8</Value>
</Quantity>


The metadata might be references to outside so they don't have to
be repeated, I don't really understand how to do references in XML:

<Quantity name="ID.Bandpass" ucd="em.bandpass">V</Quantity>
<Quantity name="ID.SurfaceBrightness" ucd="flux.sb" unit="mag/arcsec^2">24</Quantity>

<Quantity name="IsophotalDiameter" ucd="Pos.diam.isophotal">
 <Quantity ref="ID.Bandpass"/>
 <Quantity ref="ID.SurfaceBrightness"/>
 <Value>3.8</Value>
</Quantity>

In any case I don't see the role for UTYPE. Can you illustrate?

 Jonathan