convex != intersection

[Ed Shaya]

If a Convex is defined as the intersection of Constraints, as it is.  
And Constraints are defined as spherical caps.  Then, one can not 
describe a Convex by a list of points, ordered or unordered.   This is 
because the only way you have of connecting the points is by  great 
circles or small circles but the edges of spherical caps are neither.  
So, the only way to define an intersection of spherical caps is as an 
intersection  of spherical caps which makes this redundant with a 
general intersection which is already in the spec.  Therefore it should 
be removed.

Secondly, there is  no way to define a region between two values of  
declination and two values of right ascension.  Come on!!!!????

Ed


Arnold Rots wrote:

>And so, a convex is defined by an unordered list of points, whereas a
>polygon is defined by an ordered list.  A convex is a polygon -
>specifically, a convex polygon.  But polygons are allowed to be concave.
>
>  - Arnold
>
>Roy Williams wrote:
>  
>
>>A convex set is one where for every pair of points in the set, the
>>shortest path between them is completely contained in the set.
>>
>>The intersection of two convex sets is also convex by this definition.
>>
>>Perhaps the question is what is meant by a "rectangle" on the
>>celestial sphere?
>>
>>Roy
>>
>>--------
>>Caltech Center for Advanced Computing Research
>>roy at cacr.caltech.edu
>>626 395 3670
>>----- Original Message ----- 
>>From: "Ed Shaya" <Edward.J.Shaya.1 at gsfc.nasa.gov>
>>To: <registry at ivoa.net>
>>Sent: Thursday, January 29, 2004 10:12 AM
>>Subject: convex != intersection
>>
>>
>>    
>>
>>>Why is an intersection of coverage constraints called "convex"?
>>>
>>>The intersection of two rectangles may be another rectangle.  What
>>>      
>>>
>>is
>>    
>>
>>>convex about that?
>>>
>>>Ed
>>>
>>>
>>>      
>>>
>--------------------------------------------------------------------------
>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/
>--------------------------------------------------------------------------
>
>  
>