VOunits draft

[Rob Seaman]

On May 22, 2009, at 12:27 PM, Tom McGlynn wrote:

> The document doesn't seem to address how magnitudes are to be  
> handled. Are they unitless?  Do we have a 'mag' unit a la the FITS  
> standard? What about magnitudes per square arcsecond? If they are  
> unitless how do we find out that the column is a magnitude?  UCD?  
> UType?  Do we allow SI prefixes?

Or instrumental magnitudes versus absolute magnitudes.

Perhaps our interpretation of the concept of "unit" has been warped by  
all those junior high school science teachers who told us that every  
number has to be accompanied by a unit.  So we tend to regard units as  
essential metadata, when they are really just shorthand for the far  
more complex set of data and metadata (filter transmission curves,  
light time corrections, etc.) whose interpretation is required to pose  
proper scientific inferences based on empirical observations.

Note that we haven't been talking about very complex data the last few  
messages.  A list of magnitudes vs. MJD is the simplest time series  
the VO will ever see.

Rob