Boxes and Polygons in ADQL/STC. Questions and recommendation.

[Tom McGlynn]

Alberto Micol wrote:
> On 24 Oct 2009, at 20:24, Tom McGlynn wrote:
>
>   
>> Roy Williams wrote:
>>     
>>> It seems that the region specification does not support the RA/Dec  
>>> limits, or the Glon/Glat limits, etc etc. All polygon boundaries in  
>>> ADQL Region are great circles.
>>> -- Is that correct?
>>>
>>> Therefore I assume the recommendation will say that such regions  
>>> should NOT be implemented with ADQL Region, but should be  
>>> implemented directly in the SQL query like this:
>>>    RA between 200 and 210 and Dec between 20 and 30.
>>> -- Is that correct?
>>>
>>>
>>>       
>> I think so.  There is one issue that might make it nice to have a  
>> special function for this kind of box: the wrapping of longitude  
>> values.  E.g.,  if I want the 20x20 box from 350 to 10 degrees in  
>> lon and -10 to 10 in lat, the syntax is different than for the  
>> region from 330-350 in lon.  So it would be nice to have some syntax  
>> -- say rect -- which would allow users to specify
>>
>>   rect(350,10,-10,10) == rect(-10,10, -10,10) == rect(350,370,-10,10)
>>
>>     
>
> That "rect"  does not seem to be equally useful when spanning across  
> the pole...
> How do I express a rect that is centered at dec=85 and that extends 10  
> deg in dec?
>
> Al
>   

If what you want is a box with a given angular size that happens to 
include the pole, then the current ADQL box is closer to what you want.  
However in most cases I think it won't work for you because you won't be 
able to orient the box appropriately.  A box function of the form

    box(lon,lat,width,height,angle)

where the last argument is new and gives a position angle for the cross 
that defines the box would address this.  I think this would be quite a 
powerful concept and might be used quite a bit.  No one has yet 
suggested use cases where the current box function, without that last 
argument, seems especially helpful.

Generally I think boxes would tend to be small and would tend to reflect 
individual observations.  Rects, would tend to be much larger.  They'd 
be useful in defining things like, the region a telescope can view 
during a particular observing campaign, or a simple bound for the 
overall coverage of a survey.  It's certainly possible to do rects 
without defining a special function for them, but as I suggested above 
it's not entirely trivial to do right.

    Tom
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