STC Questions

[Francois Ochsenbein]

Questions and Comments about STC
--------------------------------

The comments and questions included here originate mostly in our trials to 
make use of STC in the VOTable context. No special order in the questions,
they are just numbered for convenience.

1. The Various Spherical Coordinates (2D and 3D)

   In the "CoordFlavor" element concerning spherical and unitsphere:
   different conventions are in use for the 2nd angle (latitude).
   The latitude convention measures this second angle from the equatorial 
   plane, but other conventions measure this second angle angle from one 
   of the poles (as "north polar distance" or "south polar distance"). 
   Could CoordFlavor be extended ? Polar distances have some advantages
   (always a positive value)

2. The Question of Epoch (Question (4) of the RFC page)

   The epoch of a coordinate system represents the date to which the
   spherical angles are reduced. After reflexion, it looks more logical
   to attach  the Epoch to the AstroCoords structure (section 4.6)

3. The Question of Units

   The STC defines the units as an attribute to the AstroCoords elements.
   When used in the VOTable context, a conflict may (and frequently does)
   arise between the units defined at the STC/AstroCoords and
   the units defined in the VOTable(s). One solution could be that 
   STC defines only default (suggested) units.

   But other problems with the units exist: 
   -- heterogeneous units in 3D spherical coordinates like "deg deg m"
      look ugly, and is a nonsense from a physical point of view
      (see also separation of axes below)
   -- in practice units used to express angles, their error, and angular
      sizes are different. The current schema defines a single unit
      for all these elements. By the way what is meant by 
      `The UnitSphere has default units ""' ? unitless angles ???
      radians ?

4. Question about Velocity Units

   The schema contains strange definitions for the units defined for
   velocities:
   -- one attribute "unit" which contains an angular or linear unit
   -- one attribute "vel_time_unit" which contains a time unit.
   A unit of a velocity should refer to a velocity, not a distance.
   Why can't there be a single "unit" attribute taking the values
   "km/s" or "arcsec/yr" or any valid velocity unit ? 

5. Format of the angular/time values
   For time it is possible to use a large variety of formats
   (JD, ISOtime, MJD, decimal year..) but for angle the sexagesimal
   is not allowed ? 

6. Separation of Axes

   In the VOTable context, we need to be able to refer to one axis
   of the coordinate frame. Even though VOTable has the notion of 
   groups, it seems preferable to keep the possibility of designating 
   one component along one of the axes, since some catalogs do
   contain the values on only one axis, e.g. meridian catalogs (RA
   only) or just galactic latitude (when longitude is not important).

   The current STC definitions do not allow the possibility of addressing
   a single axis of a reference frame: the XPath mechanism does not 
   permit (as far as I know) to address e.g. the 3rd component of 
   a multivalued element -- could some XML guru confirm this limitation ?
   The "utype" mechanism, based on the XPath syntax, has the same limitation 
   -- unless we decide that the "utype" extends somewhat the XPath mechanism.

   The merge of values having different physical natures into a single
   unstructured element is another motivation for separating the axes:
   the association of 2 angles and a distance (spherical coordinates),
   into a set of just 3 numbers is a source of potential misinterpretations
   and of errors (the order in which the components are given may
   differ between applications). This association in a single entity also
   requires a definition of multi-dimensional units like "deg deg m"
   which have no physical meaning.

   Separation of the 2 axes may be done in two ways:

   Way1: Sequential enumeration of the axes, each axis containing
   its properties (in a way similar to the Quantity model):
         <Position2D>
	   <Axis>
	     <Name>Longitude</Name>
	     <Value unit="deg">234.5678</Value>
	     <Error unit="deg">0.0003</Error>
	     ...
	   </Axis>
	   <Axis>
	     <Name>Latitude</Name>
	     <Value unit="deg">-0.0512</Value>
	     <Error unit="deg">0.0003</Error>
	     ...
	   </Axis>
         </Position2D>
    (Note: it could be possible to use <Axis1> <Axis2> ... instead of a 
         repetition of <Axis>)

   Way2: closer to the current definitions, each property enumerates the
   values on each axis:
         <Position2D>
	   <Name>
	     <Axis>Longitude</Axis>
	     <Axis>Latitude</Axis>
	   </Name>
	   <Value2 unit="deg">
	     <Axis>234.5678</Axis>
	     <Axis>-0.0512</Axis>
	   </Value2>
	   <Error2 unit="arcsec">
	     <Axis>1.1</Axis>
	     <Axis>1.1</Axis>
	   <Error2>
         <Position2D>
    (Note: it could be possible to use <Axis1> <Axis2> ... instead 
     of a repetition of <Axis>)
   However, such a definition means that <Axis> may be composed of a 
   String (when sub-tag of <Name>) or a Double (when sub-tag of Value2). 

   A reference to the longitude component may then be written:
   -- in Way1:
      crd:AstroCoords[@coord_system_id="GALACTIC"]/Position2D/Axis[1]/Value
   -- in Way2:
      crd:AstroCoords[@coord_system_id="GALACTIC"]/Position2D/Value2/Axis[1]

Thanks for any answer or comment to these questions !
================================================================================
Francois Ochsenbein       ------       Observatoire Astronomique de Strasbourg
   11, rue de l'Universite F-67000 STRASBOURG       Phone: +33-(0)390 24 24 29
Email: francois at astro.u-strasbg.fr   (France)         Fax: +33-(0)390 24 24 17
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