In common parlance, a "coordinate system" is surely any collection of axes
that can be used to identify points in some specified domain. I would have
thought this definition applies to all domains of any type or
dimensionality. So sure - you could use equatorial J2000 RA and Dec axes to
form a "coordinate system" to describe point on the 2D celestial sphere.
But it would be just as valid to describe a set of (ra,dec,frequency) axes
as a "coordinate system" that describes positions within the domain of a a
spectral cube.

David


On 7 February 2014 07:11, Erik Tollerud <notifications@github.com> wrote:

> In generalized_wcs/coordinate_systems_api.md:
>
> > +        def transform_to(self, other, *args):
> > +            #use affine transform
> > +
> > +
> > +    class DetectorFrame(CartesianFrame):
> > +        def __init__(self, reference_pixel, units=[u.pixel, u.pixel], name=None, reference_position="Local",
> > +                     axes_names=None):
> > +            super(DetectorFrame, self).__init__(2, reference_position=reference_position, units=units, name=name, axes_names=axes_names)
> > +            self._reference_pixel =
> > +
> > +
> > +A `CoordinateSystem` has one or more `CoordinateFrame` objects:
> > +
> > +    class CoordinateSystem(object):
> > +        """
> > +        A coordinate system has one or more frames.
>
> You're getting this use of "coordinate system" from the STC, right? This
> is yet another example of "coordinate system" being an overloaded term: the
> STC (I believe) means it to be something like "multiple coordinate frames
> mixed together to create a higher dimensional space" - that is a "system of
> coordinates" in the same sense of the word as a "system of equations". But
> that's *not* the common usage of "coordinate system" meaning something
> like equatorial J2000 ir whatever. So I think we would want to try to use
> different terminology here. (some possible, admittedly not great, ideas:
> MultiFrame, SystemOfFrames, or MultiDimCoordinates)
>
> --
> Reply to this email directly or view it on GitHub<https://github.com/astropy/astropy-api/pull/10/files#r9531363>
> .
>