SEP for the re-implementation of `sunpy.wcs` as `sunpy.coordinates`

Putting in the PR so that people can form their opinions on what I've written so far.
Fire away.
So the example specifies the location of the point in space relative to the
unknown position the observer?  The point in space is at (1100'', 0'')
helioprojected, and 1 AU from the observer?

On Fri, Sep 19, 2014 at 9:00 AM, Stuart Mumford <notifications@github.com>
wrote:

> In SEP-0005.md:
>
> > +system, where it is unusual to have information on the third spatial
> > +dimension of a coordinate, some assumptions are normally made when
> > +transforming from helioprojective to heliocentric coordinates.
> > +
> > +This assumption (as currently implemented in `sunpy.wcs`) is that
> > +during transformation, if no helioprojective radial coordinate was specified
> > +then the radial coordinate is calculated as if the coordinate point
> > +is on the solar surface.
> > +
> > +In the new coordinates submodule this assumption happens at
> > +construction time of a `HelioProjective` frame. This means that all
> > +`HelioProjective` frames are explicitly three dimensional.
> > +
> > +The third coordinate can be specified explicitly upon construction:
> > +
> > +    sc = SkyCoord(230*u.arcsec, 120*u.arcsec, 1*u.au, frame='helioprojective')
>
> ok, so the 1*au is a bit silly, because it is specifying a point deep in
> the Sun.
>
> If I make the example
>
> sc = SkyCoord(1100*u.arcsec, 0*u.arcsec, 1*u.au, frame='helioprojective)
>
> does it make a little more sense?
>
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> <https://github.com/sunpy/sunpy-SEP/pull/11/files#r17782575>.
>