VOUnits RFC

[Rob Seaman]

On Jul 29, 2013, at 7:52 AM, Tom McGlynn <Thomas.A.McGlynn at nasa.gov> wrote:

> This addresses most of Rob's questions regarding the actual format limitations, but what about his questions about whether we support single/double, what limits we need to make, ... about how these are to be interpreted.  The standard should be completely oblivious to these.

As with FITS you might be able to make the units recommendation oblivious, but questions of usage are not.

> In practice when reading these our software will read
>   1.2
> and
>   1.2345678901234567891234567801233e33
> with equal facility

So either an arbitrary precision library must be used or the handling of units must permit scale factors only as opaque literals?

> and whether the second really has vastly more precision than the first is unknowable and unaddressed by this standard.

Indeed, but this is true whether or not usage constraints are in place.

> Note that throughout I'm trying to be largely consistent with what happens in FITS with the single exception that we allow lower case e/d.

What happens in FITS is that floating point numbers are often converted to string literals that can't be reversibly converted back into single or double precision binary formats.

I don't disagree with the notion of borrowing from earlier standards, but there are implications.  Still haven't heard comments on embedding the scale factors other than as prefixes, as denominators (not an unknown usage), etc.

> P.S., in my example above, I put an embedded blank between the scale and unit.  My quick perusal of the standard suggests that this is currently illegal, but I think allowing them there at least would make things clearer, but that's not essential.

The question of embedded whitespace is broader than scale factors.

> Since d is a valid unit (for days) its use as an exponent would be disambiguated by have a following +/- or digit.

Are digits forbidden in unit names?  Are hex or other non-decimal bases permitted in scale factors?

Are there additional alpha / numeric issues for non-English usage?

Rob