Nit in VOTABLE precision attribute

[Markus Demleitner]

Hi Tom,

On Fri, Sep 29, 2017 at 10:41:49AM -0400, Tom McGlynn (NASA/GSFC Code 660.1) wrote:
> In 4.2 the text is:
> 
> The precision attribute is meant to express the number of significant
> digits, either as a number of decimal places (e.g. precision="F2" or
> equivalently precision="2" to express 2 significant figures after the
> decimal point), or as a number of significant figures (e.g. precision="E5"
> indicates a relative precision of 10???5).
> 
> So F0 would express that there a no significant digits to the right of the
> decimal point --- which seems perfectly natural.
> 
> 
> The problem is in the schema where the construct:
> <xs:simpleType name="precType">
>   <xs:restriction base="xs:token">
>     <xs:pattern value="[EF]?[1-9][0-9]*"/>
>   </xs:restriction>
> </xs:simpleType>
> 
> specifically precludes the value F0.  I believe E0 might logically
> be forbidden -- it implies no significant figures -- but maybe not.
> That's a common occurrence in actual data.  In any case I believe
> the exclusion of F0 is an erratum in the schema since it
> specifically contradicts an authoritative though perhaps
> non-normative statement in 2.3, and seems a common and desirable
> implementation of what is described in 4.2.

I agree that F0 sounds like should be allowed; E0 looks more like a
trap, but I don't think it's enough of an issue to introduce a
special case.

Also, allowing F0 will not break existing VOTables, so I'd support
writing a quick erratum
(http://www.ivoa.net/documents/DocStd/20170517/REC-DocStd-2.0-20170517.pdf,
sect 4); the way I read the specification text, it doesn't even need
to be fixed.  So, it looks to me as if the only normative thing to
change would be to replace


  <xs:pattern value="[EF]?[1-9][0-9]*"/>

with 

  <xs:pattern value="[EF]?[0-9][0-9]*"/>

in the schema.

           -- Markus