WD-DALI-1.1 polygon winding direction

[Patrick Dowler]

In STC you standardise the "on-the-left" rather than the sign of the area. OK.

And just for clariy, the common way to compute polygon area via
triangulation gives a negative value for left-handed coordinate
systems (eg celestial) and a positive value for right-handed
coordinate systems?  Is this something to mention explicitly? I note
you said "may depend"... what does that mean exactly?

Thanks for the paragraph; will add it to the next revision. I assume
that this is consistent between STC-1.x and STC-2 and I can make the
reference just to the latest STC...

Pat




On 12 September 2016 at 10:13, Arnold Rots <arots at cfa.harvard.edu> wrote:
> 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
>
>



-- 
Patrick Dowler
Canadian Astronomy Data Centre
Victoria, BC, Canada