<div dir="ltr"><div><div><div><div><div><div><div><div><div>It all depends on what expression is used for calculating the area.<br></div>The sum of determinants of successive vertex vectors requires<br></div>flipping the sign of the result for left-handed coordinates; but one<br></div>needs to be careful about what to do with cos(dec) and it will not<br></div>provide exact precision.<br></div>On the other hand, using the sum of interior vertex angles gives<br></div>a correct result independent of handedness.<br><br></div>Yes, this will not change.<br><br></div>Cheers,<br><br></div> - Arnold<br></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div>
<br><div class="gmail_quote">On Mon, Sep 12, 2016 at 1:30 PM, Patrick Dowler <span dir="ltr"><<a href="mailto:pdowler.cadc@gmail.com" target="_blank">pdowler.cadc@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">In STC you standardise the "on-the-left" rather than the sign of the area. OK.<br>
<br>
And just for clariy, the common way to compute polygon area via<br>
triangulation gives a negative value for left-handed coordinate<br>
systems (eg celestial) and a positive value for right-handed<br>
coordinate systems? Is this something to mention explicitly? I note<br>
you said "may depend"... what does that mean exactly?<br>
<br>
Thanks for the paragraph; will add it to the next revision. I assume<br>
that this is consistent between STC-1.x and STC-2 and I can make the<br>
reference just to the latest STC...<br>
<br>
Pat<br>
<div class="HOEnZb"><div class="h5"><br>
<br>
<br>
<br>
On 12 September 2016 at 10:13, Arnold Rots <<a href="mailto:arots@cfa.harvard.edu">arots@cfa.harvard.edu</a>> wrote:<br>
> No. There is no telescope that looks at the sky from the outside; it's CCW<br>
> regardless of handedness.<br>
><br>
> The following may not be brief, but I think it is clear and complete.<br>
><br>
> - Arnold<br>
><br>
> Polygons are defined by traversing their vertices in counter-clockwise<br>
> direction.<br>
> In other words, the inside of the polygon is always on the left side during<br>
> such<br>
> a traversal, regardless of whether the coordinate system is right- or<br>
> left-handed.<br>
> Note that, consequently, expressions for the area of the polygon and whether<br>
> a<br>
> particular location is contained within the polygon may depend on the<br>
> handedness<br>
> of the coordinate system; celestial coordinate systems are left-handed.<br>
> The sides of a polygon are considered part of the inside.<br>
> Also, polygon sides in spherical coordinate systems are great circle<br>
> segments<br>
> and segments have to be shorter than 180 d; if longer sides are called for,<br>
> an intermediate vertex needs to be added.<br>
><br>
> ------------------------------<wbr>------------------------------<wbr>------------------------------<wbr>-------------------<br>
> Arnold H. Rots Chandra X-ray<br>
> Science Center<br>
> Smithsonian Astrophysical Observatory tel: +1 617 496<br>
> 7701<br>
> 60 Garden Street, MS 67 fax: +1 617<br>
> 495 7356<br>
> Cambridge, MA 02138<br>
> <a href="mailto:arots@cfa.harvard.edu">arots@cfa.harvard.edu</a><br>
> USA<br>
> <a href="http://hea-www.harvard.edu/~arots/" rel="noreferrer" target="_blank">http://hea-www.harvard.edu/~<wbr>arots/</a><br>
> ------------------------------<wbr>------------------------------<wbr>------------------------------<wbr>--------------------<br>
><br>
><br>
> On Mon, Sep 12, 2016 at 12:09 PM, Patrick Dowler <<a href="mailto:pdowler.cadc@gmail.com">pdowler.cadc@gmail.com</a>><br>
> wrote:<br>
>><br>
>> So just to be clear and minimalistic, I think the only correction to<br>
>> the text in rev 3530 is that the view if from outside looking toward<br>
>> the origin. Is that correct? I will also add a reference to STC but I<br>
>> think this change would make the DALI text minimal and consistent.<br>
>><br>
>> Pat<br>
>><br>
>> On 9 September 2016 at 01:02, Marco Molinaro <<a href="mailto:molinaro@oats.inaf.it">molinaro@oats.inaf.it</a>><br>
>> wrote:<br>
>> > Hi Pat, Arnold, all,<br>
>> ><br>
>> > 2016-09-08 18:56 GMT+02:00 Patrick Dowler <<a href="mailto:pdowler.cadc@gmail.com">pdowler.cadc@gmail.com</a>>:<br>
>> >> So, should I change the wording to say "when viewed from outside the<br>
>> >> unit sphere" and add STC reference? I want to say the minimum and<br>
>> >> leave the definitive specification in the hands of STC, but if we make<br>
>> >> readers go look it up in STC they will be annoyed.<br>
>> ><br>
>> > I agree on referencing STC (even if it's not trivial because it would be<br>
>> > nice<br>
>> > to have a pointer to 2.0 or general STC), but summarizing it in DALI.<br>
>> > I'm ok with the wording, is the group agrees (warning: if no one protest<br>
>> > means agreement).<br>
>> ><br>
>> > Maybe, Arnold, can you suggest something<br>
>> > short and clear to put in DALI?<br>
>> > The reference text you replied is probably bullet-proof but too long for<br>
>> > the DALI spec (at least that's my opinion).<br>
>> ><br>
>> > Cheers,<br>
>> > Marco<br>
>> ><br>
>> >><br>
>> >> Pat<br>
>> >><br>
>> >> On 8 September 2016 at 01:00, Felix Stoehr <<a href="mailto:fstoehr@eso.org">fstoehr@eso.org</a>> wrote:<br>
>> >>> Dear all,<br>
>> >>><br>
>> >>> after some side-discussions with Marc I believe the following is true:<br>
>> >>><br>
>> >>> - STC-S declares clockwise and counterclockwise with respect to the<br>
>> >>> celestial coordinate frame (north up, east to the left as seen from<br>
>> >>> earth)<br>
>> >>><br>
>> >>> - this is consistent with the footprintfineder.py output and thus all<br>
>> >>> the MAST/ALMA/ESA spectra<br>
>> >>><br>
>> >>> - area calculations will be correct in this definition and coordinate<br>
>> >>> frame<br>
>> >>><br>
>> >>> - it is a bit counter-intuitive, because it means that anticklockwise<br>
>> >>> polygons in that definition are clockwise if you look up on the sky<br>
>> >>> from<br>
>> >>> earth.<br>
>> >>><br>
>> >>> It might be worthwile to declare in WD-DALI-1.1 the coordinate system<br>
>> >>> in<br>
>> >>> which the clockwise/anticlockwise are defined, i.e. in which system to<br>
>> >>> "hold the clock".<br>
>> >>><br>
>> >>> Best regards,<br>
>> >>><br>
>> >>> Felix<br>
>> >>><br>
>> >><br>
>> >><br>
>> >><br>
>> >> --<br>
>> >> Patrick Dowler<br>
>> >> Canadian Astronomy Data Centre<br>
>> >> Victoria, BC, Canada<br>
>><br>
>><br>
>><br>
>> --<br>
>> Patrick Dowler<br>
>> Canadian Astronomy Data Centre<br>
>> Victoria, BC, Canada<br>
><br>
><br>
<br>
<br>
<br>
--<br>
Patrick Dowler<br>
Canadian Astronomy Data Centre<br>
Victoria, BC, Canada<br>
</div></div></blockquote></div><br></div>