[meas] RFC comments - FXP

[CresitelloDittmar, Mark]

Francois-Xavier,

Thanks for posting your comments on the measurement model RFC page.
I suppose this comment/request applies to both yours and Markus' comments...

Can you make a statement about whether you see a fundamental conflict in
the model with the applications you are considering (eg: Gaia), or if the
content is compatible, but would need enhancing/refining to accommodate
your case better or more fully.

If I read your points correctly it looks like:
  + the error matrix component is very likely to be problematic as
currently modeled (seconding Markus' comments).
     So, I fully expect to push that element to a 'next' phase where the
Gaia usage might be a good thread to exercise it more directly.
   + Elliptical, Ellipsoid are compatible, but could be enhanced by the
addition of 'confidence level'.  Laurent has also mentioned this.
      This I would also prefer to push to a 'next' phase working a use-case
which contains them.

The others are more straightforward/clarifications:
   + drop one of 'stat' and 'rand'.. I'm fine with that.
   + definition of Ellipse.posAngle
      - I'd like the definition to match the common usage.. but I was under
the impression that "East of North" was a counter-clockwise direction.
Pulling from wikipedia (yeah.. I know);  "The International Astronomical
Union defines it as the angle measured relative to the north celestial pole
(NCP), turning positive into the direction of the right ascension. In the
standard (non-flipped) images this is a counterclockwise measure relative
to the axis into the direction of positive declination."
   + re: Equatorial, Galactic, Ecliptic positions..
      Yes, this is a bit orthogonal to Cartesian.  The motivation is that
at this level, I'm trying to expose the 'properties' that occur most often
in our data which we would want to identify quickly and easily.  Setting a
basis which can be expanded on in the Source properties thread.  I have
expected that "(ra,dec)" vs "(l,b)" might be an important distinction
despite them both being Spherical.  If I open a cube, catalog or whatever
and find SphericalPosition-s, then still have to go to the Frame to
determine if it's an ra/dec or l/b or lon/lat of an observatory.  That's
extra steps.  Maybe that isn't so important?

Mark
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