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Clarify and correct Units section for VOUnits #51

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Nov 20, 2023
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Clarify and correct Units section for VOUnits #51

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Clarify and correct Units section for VOUnits
mbtaylor Nov 14, 2023
6f23d83
replace erg/s with m/s in VOUnits example
mbtaylor Nov 17, 2023
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18 changes: 10 additions & 8 deletions VOTable.tex
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Expand Up @@ -1140,14 +1140,14 @@ \subsection{Units}
some physical unit,
which is specified by the {\attr{unit}}
attribute of the {\elem{FIELD}}.
The syntax of the {\em unit} string is defined in
the VOUnit specification, \citet{2014ivoa.spec.0523D};
it is basically written as a string without blanks or spaces,
where the symbols {\bf.} or {\bf*} indicate a multiplication,
{\bf/} stands for the division, and no special symbol is required
for a power.
Examples are \attrval{unit}{m2} for m$^2$,
\attrval{unit}{cm-2.s-1.keV-1} for cm$^{-2}$s$^{-1}$keV$^{-1}$,
The syntax of the {\em unit} string SHOULD conform to
the VOUnits specification, \citet{2014ivoa.spec.0523D};
this requires a string without blanks or spaces
where multiplication is indicated by the symbol ``{\tt.}'',
division by the symbol ``{\tt/}''
and exponentiation by the symbol ``{\tt**}''.
Examples are \attrval{unit}{m**2} for m$^2$,
\attrval{unit}{cm**-2.s**-1.keV**-1} for cm$^{-2}$s$^{-1}$keV$^{-1}$,
or \attrval{unit}{erg/s} for erg\,s$^{-1}$.
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I know this erg/s value didn't change with this PR, but I notice that erg is considered deprecated according to VOUnit 1.0 with the same language in the proposed VOUnit 1.1.

Should we take this opportunity to change the example to not use erg? Something like:

Suggested change
or \attrval{unit}{erg/s} for erg\,s$^{-1}$.
or \attrval{unit}{10**-7.J/s} for erg\,s$^{-1}$
(uses \literalvalue{10**-7.J} in place of the deprecated \literalvalue{erg}).

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Good point, though given that the text here is only intended to be a very brief introduction to the syntax it might be better to sidestep the status of erg by just using something like m/s instead or omit the final example altogether.

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I like the idea of simplifying or removing the last example to avoid erg. Its presence was not very important for this section.


\subsection{Unified Content Descriptors}
Expand Down Expand Up @@ -2304,6 +2304,8 @@ \subsection{Differences Between Versions 1.4 and 1.5}
\item timescales for calendar epochs.
\item positioning advice for \attr{ID} and corresponding references.
\item noting that \elem{RESOURCE} elements can contain \elem{MIVOT} blocks.
\item \attr{unit} attribute SHOULD conform to VOUnits,
and correct examples accordingly.
\end{itemize}
\end{itemize}

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