SIAP and encoding of POS input parameter

[Mark Taylor]

On Wed, 21 Jan 2009, Thomas Boch wrote:

> Dear DALers,
>
> While playing with some SIAP services the other day, I found out that
> some of them did not like the declination part of the POS input
> parameter to be started by a '+' (properly escaped as %2B).
> Example :
> http://mysiap.org/query?POS=83.6,22.0144&SIZE=0.2      is understood
> whereas:
> http://mysiap.org/query?POS=83.6,%2B22.0144&SIZE=0.2   is processed as
> an error.
>
> After checking in the SIAP document, I found nothing explicit about how
> the value of the POS parameter should be encoded.

This sort of thing should certainly be mandated explicitly in DAL and
other standards, otherwise there is no way to guarantee interoperability.
It's not good enough to leave it to the default assumptions of whatever
language the protocol is implemented in.

Several standards do spell this out.  VOTable says it with words, e.g.:

    - Single Precision Floating Point:  ...
      The representation of a Floating Point number in the TABLEDATA
      serialization is made of an optional - or +, followed by the ASCII
      representation of a positive decimal number, and followed eventually
      by the ASCII letter "E" or "e" introducing the base-10 exponent
      made of an optional - or + followed by 1 or 2 digits. The number
      must be within the limits of the IEEE floating-point definition
      (around ±3.4·1038; numbers with absolute value less than about
      1.4·10-45 are equated to zero); the default representation of a
      null value is an empty cell (see VALUES definitions above), and
      the special values "+Inf", "-Inf", and "NaN" are accepted.

and SAMP uses BNF:

      <digit>        ::= "0" | "1" | "2" | "3" | "4" | "5" | "6"
                       | "7" | "8" | "9"
      <digits>       ::= <digit> | <digits> <digit>
      <float-digits> ::= <digits> | <digits> "." | "." <digits>
                       | <digits> "." <digits>
      <sign>         ::= "+" | "-"

      <SAMP float>   ::= [ <sign> ] <float-digits>
                         [ "e" | "E" [ <sign> ] <digits> ]
          A floating point value is encoded as a mantissa with an optional
          preceding sign followed by an optional exponent part introduced
          with the character "e" or "E". There is no guarantee about the
          largest or smallest values which can be represented or about
          the number of digits of precision which are significant, since
          these will depend on the processing environment at decode time.

It would make sense for these things to be consistent across all the
IVOA standards, but different contexts have different requirements or 
obvious implementations; for instance something which is evaluated or
validated in the context of an XML schema should most obviously use
the XML Schema definition, but this might impose an unnecessary burden
in non-XML-based contexts.

The main thing is that it is spelled out in each case, either explicitly
or by reference to some other standard.

Mark

-- 
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/