units symbols

[derriere at newb6.u-strasbg.fr]

Dear data modelers,

I attach to this mail a comparison of the symbols used
for units in different sources: IAU recommendations,
VizieR, HEASARC and the GNU software Units.

The reason for this was to try to study to which extent
the representation of units was compatible among those
various sources, and see if we can solve the issue of
representing units in the VO with a minimum cost. You'll
find a set of URLs at the end of the spreadsheet for these
sources.

For the IAU, I used symbols in Tables 1, 2, 5, and also
table 6 which contains deprecated ones.

I'm *not* discussing here the issue of multiples/submultiples
(see IAU table 4, HEASARC table 3),
nor the rules for composition of symbols into a complex unit
expression (e.g. square meters written as m2, m^2 or m**2).

Note that the symbol expressions are CASE SENSITIVE (h and
H are different!).

I outlined in the spreadsheet:
green: same symbol everywhere
yellow: only trouble with GNU
gray: upper/lowercase difference
red: discrepancy

The first conclusion is that most of the basic symbols are
identical in the 4 compared sets. There are only 2 discrepancies:
- rad is not recognized for radians in GNU Units
- the unit for electric resistance is  Ohm ohm Omega ?

As we go to more exotic quantities, I noted a few issues
along the way:
- The IAU recommendation for year symbol (a) is only used in VizieR; 
the other use yr
- The definition of the year itself is ambiguous, e.g. between VizieR 
and GNU (how many seconds, julian or not?)
- IAU lists symbols I've not seen in VizieR or at HEASARC and did not 
list here: c for «cycle », and the deprecated: mu, cc, Gal, E, Oe
- the above 'c' is ambiguous with speed-of-light!
- Heasarc also uses: Crab, chan, bin, voxel
- GNU knows very little about astronomy-specific things


   Do you think this kind of comparison could be a starting point
for defining a common unit representation for the VO?

Sebastien.

PS: the 2 attached file are identical: one for OpenOffice 2.0,
the other for MS-Excel.

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