<div dir="ltr"><div><div><div><div><div>Thomas pretty much said what I was going t say.<br></div>MOC images are handy to look at, but if you want to look at coverage superimposed on, e.g., DSS, you want to see the outline of the footprint.<br>Besides, if you get beyond order 13 (a far cry from 29), the files get very bulky and the response very sluggish.<br><br></div>The issue of small circle sectors (as opposed to great circle sectors) is a bit of a pain. STC can handle it, but I don't think anyone (with the possible exception of SDSS) has ever used it, much less implemented. Great circles are nice ad they transform to straight lines in tangent projections, but they are not helpful for drift scan surveys or RA,Dec boxes.<br><br></div>Polygons are traversed CCW (i.e., the inside is always on your left) when looking at the celestial sphere from the origin.<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, Dec 4, 2017 at 4:57 AM, Thomas Boch <span dir="ltr"><<a href="mailto:thomas.boch@astro.unistra.fr" target="_blank">thomas.boch@astro.unistra.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Markus, Felix,<br>
<br>
To me, STC-S footprints and MOCs are two different, complementary objects, and we need both.<br>
<br>
STC-S footprints are useful when a client wants to draw the (mostly) accurate outline of an observation.<br>
MOCs provide a discretization of the coverage. They are great for fast coverage comparison/intersection/union and to query efficiently a database for a given coverage on the sky.<br>
<br>
Yes, you can algorithmically compute the outline of a MOC (Aladin Desktop does it). But that will only be an approximation. Plus, the size (in bytes) of the MOC will be 1 or 2 orders of magnitude greater than the initial STC-S description.<br>
<br>
--<br>
Thomas<div class="HOEnZb"><div class="h5"><br>
<br>
Le 01/12/2017 à 09:48, Markus Demleitner a écrit :<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Felix,<br>
<br>
On Wed, Nov 29, 2017 at 10:27:32AM +0100, Felix Stoehr wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Question 2: Assuming some tool support (i.e., "STC-S-to-MOC<br>
converter", and MOC support in their backen databases), how would<br>
current STC-S providers feel about migrating to MOCs on a time frame<br>
of a few years?<br>
<br>
</blockquote></blockquote>
[...]<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
ALMA can certainly compute MOCs and has actually been planning to do so.<br>
My personal impression is that however the MOCs serve a different<br>
purpose than our STC-S footprints. MOCs: fast x-match, high-order<br>
display in AladinLite. STC-S footprints: exact outline and precise<br>
display in AladinLite.<br>
</blockquote>
I don't think precision is a major issue here -- at level 29, the<br>
typcial extent of a healpix cell is of the order 0.5 mas. Even in<br>
the Gaia age, that, I believe, should go a long way.<br>
<br>
It is true, though, that MOCs hide some physics; it is, I suspect,<br>
virtually impossible to reconstruct that a certain coverage, for<br>
instance, has been produced as a union of n distinct circles, and...<br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
This seems also to be what ESA sky or ESO are doing with the new<br>
interface. We also want that users can select individual footprints by<br>
clicking on the boundary, which is a use-case that I am not sure how it<br>
could be supported by MOC.<br>
</blockquote>
...finding the outline of a MOC is probably non-trivial (though it<br>
sounds like something that's algorithmically feasible).<br>
<br>
Then again, I'm not convinced if letting people go for outlines<br>
actually is a good solution for the UI problem of letting people<br>
select one of several overlapping shapes; after all, it's perfectly<br>
possible that several of these shapes have the same outline, at least<br>
at a given resolution, and then you're back at having to be explicit<br>
about the stack again.<br>
<br>
So, for this particular problem I'd say going for, for instance,<br>
popups that let people select which of the set of shapes below the<br>
pixel in question they'd like to use would be preferable, and that's<br>
at least as straightforward with MOCs as with explicit geometry<br>
(whether written in STC-S on in some sanitation of it).<br>
<br>
That doesn't mean there aren't (other) use cases for explicit<br>
geometries. But it'd be great to understand them before going for a<br>
sanitation/standardisation of the TAP 1.0 appendix.<br>
<br>
-- Markus<br>
</blockquote>
<br>
</div></div></blockquote></div><br></div>