[QUANTITY] The quandary

[Roy Williams]

> Yes, I see there ia a lot of overlap. How are numerical parameters
> represented within UCDs? If you had a UCD for, say, a topocentric radio
> velocity value, where would the rest frequency, the observers
> geocentric longitude and latitude, the date, etc, get stored? These are
> all needed before you can compare the value with other values with the
> same UCD.

UCD has mostly been used in the context of VOTable. There would be a group
of linked quantities, maybe something like this. XML has been simplified for
clarity.

<GROUP name=v ucd=VELOC>
  <FIELD ucd=VELOC_LSR/>
  <FIELD ucd=OBS_FREQUENCY/>
  <PARAM ucd=POS_EARTH_LON value="120.0"/>
  <PARAM ucd=POS_EARTH_LAT value="32.0"/>
</GROUP>

1. When you use FIELD, the implication is that each record of the table has
different values. Therefore this GROUP implies two columns. I have assumed
all the observations are from the same place, hence the PARAMs.

2.  The UCD VELOC_LSR means "Radial Velocity Respect to the Local Standard
of Rest". That was the closest I could find to your "topocentric radio
velocity".

3. Maybe you can explore further, there is a nice tool here for finding
appropriate UCDs:
http://cdsweb.u-strasbg.fr/UCD/assignateur/