SIA 2.0 POS parameters

[Walter Landry]

Hello Everyone,

I have been looking at the draft for SIA 2.0 and I have a few comments
about the POS parameters.

1) POS uses spaces as semantically important elements as in

     POS=Circle 10 10 1

   This makes it annoying to create URL's because you must URL encode
   the spaces as '+'.  Otherwise my web server (Apache) will strip out
   the rest of the parameters.  If we instead used ADQL'ish syntax

     POS=Circle(10,10,1)

   with explicit, visible separators, that is not as prone to being
   mangled.

2) POS has no way to specify the reference frame.  I think that should
   be part of the spec, with the default being ICRS.  Again, straight
   from ADQL

     POS=Circle('GALACTIC_CENTER',10,10,1)

   I know that some galactic people here would really like that, and I
   recall a planetary person asking for it at the last meeting.  It
   would also mean that I can copy and paste coordinates between ADQL
   and SIA.

   This would also make the Range geometry more useful.  There are not
   that many things that align themselves along lat/lon lines in ICRS.
   Though to be honest, I would be happy enough to get rid of Range.
   Polygons and Box (see below) would cover most of the use cases.

3) Why isn't there a Point geometry?  I can specify a circle with
   radius zero, but that is awkward and un-intuitive.

4) There should really be a Box geometry that lets you specify a
   center, width, and height.  Otherwise, clients have to do annoying
   math to get proper boxes near the poles.  For the queries we serve,
   getting images covering a box is a very common operation.

5) Winding should be specified for polygons.  Making the interface too 'smart'
   leads to surprising outcomes.

Cheers,
Walter Landry