WD-DALI-1.1 polygon winding direction

[Arnold Rots]

No. There is no telescope that looks at the sky from the outside; it's CCW
regardless of handedness.

The following may not be brief, but I think it is clear and complete.

  - Arnold

Polygons are defined by traversing their vertices in counter-clockwise
direction.
In other words, the inside of the polygon is always on the left side during
such
a traversal, regardless of whether the coordinate system is right- or
left-handed.
Note that, consequently, expressions for the area of the polygon and
whether a
particular location is contained within the polygon may depend on the
handedness
of the coordinate system; celestial coordinate systems are left-handed.
The sides of a polygon are considered part of the inside.
Also, polygon sides in spherical coordinate systems are great circle
segments
and segments have to be shorter than 180 d; if longer sides are called for,
an intermediate vertex needs to be added.

-------------------------------------------------------------------------------------------------------------
Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------------------------------------------


On Mon, Sep 12, 2016 at 12:09 PM, Patrick Dowler <pdowler.cadc at gmail.com>
wrote:

> So just to be clear and minimalistic, I think the only correction to
> the text in rev 3530 is that the view if from outside looking toward
> the origin. Is that correct? I will also add a reference to STC but I
> think this change would make the DALI text minimal and consistent.
>
> Pat
>
> On 9 September 2016 at 01:02, Marco Molinaro <molinaro at oats.inaf.it>
> wrote:
> > Hi Pat, Arnold, all,
> >
> > 2016-09-08 18:56 GMT+02:00 Patrick Dowler <pdowler.cadc at gmail.com>:
> >> So, should I change the wording to say "when viewed from outside the
> >> unit sphere" and add STC reference? I want to say the minimum and
> >> leave the definitive specification in the hands of STC, but if we make
> >> readers go look it up in STC they will be annoyed.
> >
> > I agree on referencing STC (even if it's not trivial because it would be
> nice
> > to have a pointer to 2.0 or general STC), but summarizing it in DALI.
> > I'm ok with the wording, is the group agrees (warning: if no one protest
> > means agreement).
> >
> > Maybe, Arnold, can you suggest something
> > short and clear to put in DALI?
> > The reference text you replied is probably bullet-proof but too long for
> > the DALI spec (at least that's my opinion).
> >
> > Cheers,
> >      Marco
> >
> >>
> >> Pat
> >>
> >> On 8 September 2016 at 01:00, Felix Stoehr <fstoehr at eso.org> wrote:
> >>> Dear all,
> >>>
> >>> after some side-discussions with Marc I believe the following is true:
> >>>
> >>> - STC-S declares clockwise and counterclockwise with respect to the
> >>> celestial coordinate frame (north up, east to the left as seen from
> earth)
> >>>
> >>> - this is consistent with the footprintfineder.py output and thus all
> >>> the MAST/ALMA/ESA spectra
> >>>
> >>> - area calculations will be correct in this definition and coordinate
> frame
> >>>
> >>> - it is a bit counter-intuitive, because it means that anticklockwise
> >>> polygons in that definition are clockwise if you look up on the sky
> from
> >>> earth.
> >>>
> >>> It might be worthwile to declare in WD-DALI-1.1 the coordinate system
> in
> >>> which the clockwise/anticlockwise are defined, i.e. in which system to
> >>> "hold the clock".
> >>>
> >>> Best regards,
> >>>
> >>> Felix
> >>>
> >>
> >>
> >>
> >> --
> >> Patrick Dowler
> >> Canadian Astronomy Data Centre
> >> Victoria, BC, Canada
>
>
>
> --
> Patrick Dowler
> Canadian Astronomy Data Centre
> Victoria, BC, Canada
>
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