TAP RFC [xtype]

[Patrick Dowler]

On Thursday 17 September 2009 14:08:58 Markus Demleitner wrote:
> > The use of point above is for databases that really expose geometry
> > types. For those with simple scalar columns for coordinates, those would
> > be done with two columns (FIELDs); both columns would ref="something" to
> > specify the same coordinate system, and the xtype would be an extension
> > of the above to signify which is longitude and which is latitude, eg
> >
> > name="ra" xtype="stc:Position2D.C1"  datatype="double"
> > name="dec" xtype="stc:Position2D.C2"  datatype="double"
> >
> > Caveta: I don't know if the .C1 is correct, or if it should be / or some
> > other connector (TBD).
>
> Argh.  Please don't.  While I think this (a point is just a pair of
> numbers) is what things should have looked like, this unfortunately
> is askew with ADQL that requires the coordinate system.  Actually,
> ADQL has its own idea of how its geometries should be encoded on
> output.  So, now another format.  And what about ADQL CIRCLEs,
> POLYGONs, etc?

I don't actually think separate columns for coordinate values really need an 
xtype at all since the standard VOTable datatype is sufficient for both reading 
(upload) and writing (result) tables. It was just an unnecessary extrapolation 
:-) 

> > POINT : xtype="stc:Position2D"  datatype="double"  arraysize="2"
> >
> > For point, the content is the usual comma-delimited pair of numeric
> > values, and use a ref="something" to connect it to a coordinate system
>
> Well, according to the serialization rules of VOTables, this is not
> comma-delimited but either whitespace-delimited (tabledata encoding)
> or whatever else.  But that's a fine point.

So aside from the delimiter, is this a viable way to describe a column full of 
coordinate pairs (POINTs in ADQL)? It probably requires a ref="the_coord_sys" 
in order for the values to be as explicit as POINT constructs in ADQL (but 
still not repeating the coordinate system in every table cell).

Alternatively, what alternative with an xtype="stc:???" can explicitly say 
that all the values are points?

-- 

Patrick Dowler
Tel/Tél: (250) 363-0044
Canadian Astronomy Data Centre
National Research Council Canada
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Centre canadien de donnees astronomiques
Conseil national de recherches Canada
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Victoria (C.-B.) V9E 2M7