[Units] : (Was: Re: SED Data Model: Questions and Comments

[Brian Thomas]

On Wednesday 16 February 2005 01:38 pm, David Berry wrote:
> 
> On Wed, 16 Feb 2005, Brian Thomas wrote:
> 
> >  I have no problem with this, and would go for this point of view because it provides
> >  a many to one translation layer.. e.g.
> >
> >  units system A ---> DM Units model --> units system B
> 
> I guess you were referring to the dimensional analysis model as the
> "common" moddel in the middle. This raises another question I had.
> Whereas the conversion from string to dimensional analysis is well
> defined, is the inverse conversion well defined? That is, given a
> dimensional analysis and a scale factor, does it imply a single units
> string? If not, I guess many of the possible strings will be equivalent,
> but some may not be. Again the issue of dimensionless units such as
> steradians, degrees, pixels, etc, could mess things up. If you convert
> "Jy/sr" to dimensional analysis form and then convert back to string do
> you get "Jy/sr" or "Jy". These are obviously very different.

 Good point and true. There are such things as "dimensionless" units 
 (e.g. "angle" and "number of <thing>") which *do count* as "dimensions".

 If you look carefully at systems of units, you will find (I think) two
 or so dimensionless dimensions exist (I may have missed a third..
 Ed might remember). Nevertheless, these would have to be recognized
 and handled appropriately by the bridging parsing software to the common 
 model. (and, obviously, the common model would have to encompass 
 these "dimensionless" dimensions as well)

 =b.t.

> 
> David
> 

-- 
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|                                     
| Dr. Brian Thomas                    
|                                     
| Dept of Astronomy                   
| University of Maryland-College Park 
|
| Phone: (301) 405-2312               
| Fax: (301) 314-9067                 
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