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

[Brian Thomas]

On Wednesday 16 February 2005 12:32 pm, Patrick Dowler wrote:
> I think the big win for dimensional analysis is that when you write code to 
> handle unit strings, you essentially parse a bunch of ugly stuff using lookup
> tables and trun it into a form you can work with  - the dimensional analysis
> exponents and scale factors - then you use that to convetrt values to your 
> preferred unit system.

 I guess I get confused when people talk about a procedure/process as a model.
 What you are saying (I think) is that you want the model to capture the dimensional
 analysis aspect of units in a DM Units model, yes?

 Does this model contain the original string which spawned it, or can it 
 standalone?

 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

 Where the arrows are parsing software written respectively by owners of systems A, B.
 As you point out, this is probably a sub-set of the "full" DM Units model, and fairly 
 straitforward to write. The DM would provide the common model, and there would be one 
 or more packages (one would hope) to handle the units conversions, calculations, etc 
 on information contained in the DM model. 

 This does seem to be the smarter way to implement units.

> David writes:
> The issue is ensuring that the model includes all the necessary meta-data
> needed to convert *from* MKSA units to other more esoteric systems. For
> instance, if all flux density-like data is required to be stored in
> W/m^2/Hz (or do I mean W/m^2/Angstrom?) within the DM but the end user
> wants to see antenna temperature, then the DM needs to include all the
> necessary efficiency factors, etc, needed to do this conversion. Likewise,
> converting to/from magnitudes.

 I agree this is an important issue, but is it really all that insurmountable? 
 Perhaps we can target a 90% functionality in the first-cut DM model, and 
 allow that some issues/functionality will have to be resolved/added in a 
 further iteration.

> Then what if the meta data needed to convert from your estoteric system to
> the standard DM system are only poorly known? It is surely possible that
> useful science could be done in the esoteric system, without ever
> converting to the standard system. 

 Again, I agree this is important issue to solve. But I question that if one has 
 such an esoteric system of units, then I wonder of what general value is the 
 data to the VO as a whole? Nevertheless, perhaps the issue can be somewhat
 resolved by allowing a 2-tier system:

  Essentially there are 2 allowed models for units:

 1. a common DM model of units as mentioned above that is readily parseable
 and useable for calculations.

 and

 2. DM "string model" which contains only the string of units and meta-data which
 identifies the system to which it belongs.

 Eventually one would hope to convert the later units into the former.

 =brian


-- 
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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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