STC-S Regions (was: Boxes and Polygons)

[Alberto Micol]

Hi Arnold,

On 26 Oct 2009, at 22:23, Arnold Rots wrote:

> I have been trying all day to find time to wade through the email
> deluge of the weekend. I think I have collected the relevant issues
> and unanswered question that concern STC-S's definition of Box and
> Polygon.
>
> In discussions with Markus Demleitner and David Berry, I have been
> working over the past year on an update of the STC-S note.
> you will find the current version at:
>
>    http://hea-www.harvard.edu/~arots/nvometa/STC/STC-S-20091030.pdf
>
> Several restrictions have been removed, most notably compound regions.
> And there are two remaining issues to be worked out:
> - STC-X does not allow more than one PositionInterval, for subtle
> reasons that have to do with XSD syntax; but there may be a way out.
> - STC-S spherical Polygons are limited to great circle sides; adding
> small circles would not be too difficult, if it were not for the fact
> that if one were to allow a pole to be specified things get very
> messy; maybe only small circles with default pole should be allowed?
> Note, though, that small circles can be specified if one uses
> Convexes; at the same time, though, it means that this is a case
> where STC-S does not allow full and transparent translation between
> Convex and Polygon (as opposed to STC-X).
>
> Inside and outside:
> Tom (I think) is right that users may not pay attention to the
> ordering of their vertices, but somehow they must specify what it is
> that they mean, and going around the area counter-clockwise is as good
> as any. This really should be taken care of in the user interface.
> Note that this is an issue for Polygon, not for Convex - the operation
> there is more involved than simply reversing the order.
>
> Small circles
> Francois is correct: reversing the order of the vertices without
> moving the SmallCircle elements changes not only the meaning of inside
> and outside, but also the specification of the small circles. The
> reason is simply that the Vertex elements are a combined specification
> of vertex and side.
> Note that this is not an issue when using Convexes.
>
> Boxes:
> Read the definition carefully. It says that the arms of the cross must
> be parallel to the coordinate axes AT THE CENTER POSITION.

Yes, but it is not said that the arms themselves must be on great  
circles.
Hence I could in principle choose to place the arms on any of the  
infinite
circles all parallel to the equator at the center position.

> And it says
> that the sides are great circles perpendicular to the arms of the
> cross AT THEIR END POINTS.
> It also says that the Box is a special case of a Polygon and that the
> vertices of a Polygon shall be less than 180 deg apart.

As a very little suggestion, it might be useful to repeat in the BOX  
definition
what is specified in the polygon definition, that is. the fact that
"vertices need to be less than 180° apart in both coordinates".

Thanks Arnold for the precious clarifications,
Alberto


> The intent is indeed, as John Good and other remarked, for the Box to
> be used to specify image bounds - i.e., rather small Boxes.
>
> For "coordinate rectangles" there is little need for a special region
> definition, since it is so easy to specify in SQL:
>  ... where ra between 60 and 70 and dec between 30 and 40
> In STC-S one would use a PositionInterval.
>
>
> I think this covers the issues that have been raised in the last few
> days. Let me know if I missed anything - I guess I should say: let me
> know what I missed. I have not checked the coordinate transformations
> that were suggested.
>
> Cheers,
>
>  - Arnold
>
> --------------------------------------------------------------------------
> 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 head.cfa.harvard.edu
> USA                                     http://hea-www.harvard.edu/~arots/
> --------------------------------------------------------------------------