SSAP and images

[Anita M. S. Richards]

On Tue, 1 May 2007, Guy Rixon wrote:

> Anita,
>
> by my reading of the SSAP spec, the position encoding is part of the Spectrum 
> data model:
>
> Coverage.Location.Sky.Value pos.eq RA and Dec deg, R
> (or sexagesimal string)
>
> If I understand this correctly, each column of the VOtable returned by SSA is 
> a node in the Spectrum data model, with an associated Utype. This position 
> node is defined as a coordinate pair, so the column isn't a scalar, numeric 
> type. It would have to be either a string or an array of float or double. 
> Would the array form work for you? I guess the string form covers the case 
> where the coordinates are sexagesimal.
-----
On Tue, 1 May 2007, Doug Tody wrote:

> Hi Anita -
>
> SSAP and SIAP both return positions as type double.  I think you need to 
> address this at the level of applications integration.  (Also, this just 
> goes to show, that although we may know the type of an object, that 
> doesn't say much about what to do with it).

Thanks to all for the comments.

I have just done a quick comparison, and in fact some SSAP votables do 
have RA and Dec (in decimal degrees) as type Double, so that the 
positions can be easily plotted in TopCat, as can most SIAP VOTables.  So 
I think that there are two issues:

Firstly, that SSAP and SIAP tables can have position coordinates in a form 
which can be plotted by TopCat, but don't always.  I don't know whether 
this is because the standard is insufficiently defined, or whether it is 
OK but open to misinterpretation (those pesky users again) or because 
some archives get it plain wrong (as a data provider, I know I commit all 
three sorts of error...).  I _think_ that is covered by 
what Guy says - and, following from that, if coordinates are in decimal 
then they are an array of two doubles or floats; if in sexagesimal, then 
strings?.

Secondly, it is up to the tool authors to provide a means of overplotting 
and I will take that up separately.

Thanks very much

Anita






> On 1 May 2007, at 17:07, Anita M. S. Richards wrote:
>
>> 
>> Hi people,
>> 
>> Today we were doing a demo at the E-Science North West open day in 
>> Manchester and someone asked 'can you display the positions of spectra on 
>> top of images'.  So I used AstroScope to get some spectra and images and 
>> sent an image to Aladin, and sent the SSAP VOTable to Aladin and TopCat. 
>> Two problems emerged:
>> 
>> The position is given in decimal degrees but type is string.  If I change 
>> to float, TopCat will plot the positions.
>> 
>> However, even then, Aladin will only offer to display it in a spectral tool 
>> - it won't plot the positions.
>> 
>> Probably we ahve discussed this and there are good reasons for it not 
>> working? Or is it something to do with retrieving SSAP stuff via 
>> AstroScope? But otherwise, it seems easy to fix and would please users.
>> 
>> Thanks
>> 
>> Anita
>> 
>> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>> Dr. Anita M. S. Richards, AstroGrid Astronomer
>> MERLIN/VLBI National Facility, University of Manchester, Jodrell Bank 
>> Observatory, Macclesfield, Cheshire SK11 9DL, U.K. tel +44 (0)1477 572683 
>> (direct); 571321 (switchboard); 571618 (fax).
>
>