WD-DALI-1.1 polygon winding direction

[Patrick Dowler]

I checked more closely and while my own code for CAOM doesn't care
about the winding direction* it does detect it correctly as CCW for A
> 0, but when viewing the tangent plane from the outside... so what I
wrote in the last sentence is wrong and my implementation agrees with
what Felix says above. I think that also agrees with STC and a
reference to STC is appropriate in this section of DALI.

As for the agreement, we didn't agree on the details, just that we
would pick a winding direction and allow polygons larger than half the
sphere.

As for impact, we store polygons with both winding directions in our
CAOM database so I will have to recompute ~half of them. I'm fine with
that.

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.

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