Discussion on VOUnits

This page is created to discuss the VOUnits 1.0 document, and to find a consensus on undecided issues, by listing pros and cons on each item. For each item on the page, various options are presented, and the community is encouraged to express opinions in order to reach a consensus, by appending your wikiname to the preferred option, and/or commenting.

See Unity library notes.

Symbols

Electric resistance

The IAU recommends to use the upper-case greek letter Omega, but this is not allowed with the adopted character set for VOUnits. OGIP uses ohm (lower-case o), while StdCats and FITS recommend Ohm (upper-case o).

The upper-case version seems more appropriate (refers to the name of Georg Simon Ohm).

Options for VOUnits :

1. Accept both ohm and Ohm. pro: remains compatible with all current use. con: adds complexity to parsing.
2. Only allow Ohm. pro: easier parsing. con: translation needed for OGIP (but is this unit really used in astronomy?).

Discussion:

Micro-arcseconds

This is not yet widely used, but it will probably be the case with future astrometric missions like GAIA. We have arcsec, and mas (while IAU recommends nanoradians nrad). Options for VOUnits :

1. Follow only IAU style: nrad, prad...
2. uas
3. uarcsec

Discussion:

Celsius

Do we keep a degC symbol?

Discussion:

Dropped deprecated symbols (Table 5)

Any objections to removing these from the standard ? dyn, cal, bar, atm, Gal, eotvos, gamma, oersted

Discussion:

Other symbols (Table 6)

Is it OK to reuse the FITS 2010 notations? (FITS v3.0, section 4.3, W.D. Pence et al., A&A 524, A42, 2010. doi:10.1051/0004-6361/201015362)

Discussion: I think we should more-or-less adopt the FITS specification, clarified where appropriate, and adjusted where necessary. -- NormanGray

More than one solidus

The FITS specification, and the draft VOUnit specification both say "The IAU style manual forbids the use of more than one slash (/) character in a units string. However, since normal mathematical precedence rules apply in this context, more than one slash may be used but is discouraged." There's no way of expressing 'discouraged' in a grammar -- something either is a valid string or it isn't. I suggest that the VOUnit grammar simply forbid multiple solidi. -- NormanGray - 20 Dec 2011

Discussion:

Fractional powers and brackets

I wasn't able to deduce from the FITS spec whether m^1.5 was legal or not, because the power is not surrounded by brackets. I think it's meant to be illegal (and the grammar in the Unity library reflects this), but the VOUnit spec should be more precise here. I don't think it much matters which way we decide. -- NormanGray - 20 Dec 2011

Discussion:

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Special cases

Quantity with no unit

For example a character string or a dimensionless ratio. We suggest to use an empty string. Other possibilities are one or 3 dashes, or blanks.

Discussion:

Percent

This means a factor 0.01, with no unit. Do we allow "%"?

Discussion:

Unknown unit

What to do when we know the unit is not known?

1. Use a question mark: ?
2. Use an empty string
3. Other notation?

Discussion:

Does this have to be specified in the VOUnit document? I don't think so, and believe it can be a library matter.

I suggest we simply define three conformance levels: that a unit string is 'as recommended', 'is acceptable' (meaning that it uses only recognised but not necessarily recommended units), and 'is parseable' (meaning that it conforms to the grammar, but that there are either unrecognised units or units used in an disrecommended way, such as with inappropriate SI prefixes). Since the document is defining a grammar rather than an API, it's not reasonable for it to define error behaviour. -- NormanGray - 20 Dec 2011

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Mathematic expressions

Any objections to following the FITS guidelines?

Regrets that VOUnits don't include trigonometric functions?

Discussion:

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AOB ?