dimensionless units

[David Berry]

Jonathan,


> I'm just saying, even if we
> have redundant metadata (unit and sifac/siunit) we don't need redundant
> DIFFERENT string syntaxes. If we're going to have a string syntax for
> the "dimensional" analysis, why not use the SAME syntax that we use
> for general unit analysis, since then other people's clever unit software can handle
> either UNITSCAL/UNIT or SISCAL/SIUNIT with the same parser.

I think one of Pedro's points was that we do not currently have one string
syntax, but many. His proposal as I understand it is to include old,
potentially non-standard unit strings for the benefit of old readers, and
in addition include standardised information for the benefit of new
readers. As to what form the new standardised information should take, I
tend to agree with you that a "conventional" unit string would be better
since it retains information about dimensionless units such as rads, degs,
arcmins, steradians, etc, which would presumably be lost in a strict MLT
description. It also allows the continued use of common things like
"log(Hz)".

> If clever unit software becomes widespread and standardized we maybe
> eventually can drop the SI.. version.

Surely, it's not the software which needs to be standardised but the
syntax for units strings - but FITS-WCS paper I already does that. And
software does not need to be *that* clever. Handling FITS-WCS paper I
strings is certainly not trivial, but its easier than a lot of the other
software issues being raised by the DM group! Just to show it can be done,
I've put up a CGI script which uses the AST library to determine a Mapping
between any two FITS-WCS paper I units strings (various common variants
are also allowed). Go to http://www.starlink.ac.uk/ast/ and look for
"Units Facilities".

David