An STC "when" and "where" example

[Arnold Rots]

Roy,

What you have there is not a valid document.
I'm a bit puzzled, though, why someone would want to use VOEvent to
just transmit a position - you might as well send e-mail saying:
"RA, Dec: 148.88821 69.06529"
I would have thought that providing a time, wavelength, observer's
location, and errors would be essential.

STC-X does not allow defaults, in general, the reason being
demonstrated by what you wrote: those defaults sounded very reasonable
to you, but I would have chosen rather different ones.  Since the VO
intends to tie together many subcommunities, the use of defaults is
dangerous.
Giving minimal, sketchy information may be fine for some of your
clients because they know exactly what you mean or they don't require
great accuracy, but there may be other potential clients who cannot do
anything with it because there isn't enough information; the planetary
community is an obvious example.

As to your stripped-down version, I can make a similar one that is
valid by using XIncludes and I have appended that one at the end of
this message.  Three more lines than yours, one of which I promise to
get rid of in the next version of the schema.  Any parser that feels
like it (or knows what they say) can skip the included documents which
are going to be part of a standard IVOA library.
And, yes, of course I could have gotten rid of the spectral frame if
we are only dealing with positions.

Spectral and redshift frames are in there because these coordinates
are closely interwined with space and time.  Clearly, you must know
the motion of the observer wrt to the source in order to interpret
these coordinates correctly.

Does this help?

  - Arnold


Roy Williams wrote:
> Arnold
> 
> I have stripped out some of your data to leave only what is below. It just a postion, 
> and has RA and Dec in ICRS coordinate system. Please can you tell me if this is a 
> valid STC document? If so, it would seem quite easy to parse. If not, what are the 
> other parts that must be in it?
> 
> Roy
> ------------------------------------------------------
> 
> Specifically I removed:
> -- Observatory location, I assume this defaults to the center of the Earth.
> -- Time of observation, I assume this defaults to 1/1/2000.
> -- I changed coord_system_id="ICRS-TT-WAVELENGTH-TOPO" to be just "ICRS", is that OK?
> -- Error on the position, I assume it defaults to zero error.
> -- Wavelength information, please can you explain why this is in with the space-time 
> coordinate systems?
> 
> And here is the minimal document to say RA=148.88821, Dec=69.06529:
> 
> <?xml version="1.0" encoding="UTF-8"?>
> <ObsDataLocation
> xmlns="http://www.ivoa.net/xml/STC/stc-v1.20.xsd"
> xmlns:crd="http://www.ivoa.net/xml/STC/STCcoords/v1.20"
> xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
> 
>     <ObservationLocation ID="TaxTimeEvent">
>         <crd:AstroCoords coord_system_id="ICRS">
>             <crd:Position2D unit="deg">
>                 <crd:Name>RA,Dec</crd:Name>
>                 <crd:Value2>148.88821 69.06529</crd:Value2>
>             </crd:Position2D>
>         </crd:AstroCoords>
>     </ObservationLocation>
> </ObsDataLocation> 
> 
--------------------------------------------------------------------------
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/
--------------------------------------------------------------------------


Stripped-down valid version:

<?xml version="1.0" encoding="UTF-8"?>
<ObsDataLocation 
xmlns:xi="http://www.w3.org/2001/XInclude" 
xmlns="http://www.ivoa.net/xml/STC/stc-v1.20.xsd" 
xmlns:crd="http://www.ivoa.net/xml/STC/STCcoords/v1.20" 
xmlns:reg="http://www.ivoa.net/xml/STC/STCregion/v1.20" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://www.ivoa.net/xml/STC/stc-v1.20.xsd http://hea-www.harvard.edu/~arots/nvometa/v1.2/stc-v1.20.xsd">
    <xi:include href="http://hea-www.harvard.edu/~arots/nvometa/v1.2/KPNO.xml"/>
    <ObservationLocation ID="TaxTimeEvent">
        <xi:include href="http://hea-www.harvard.edu/~arots/nvometa/v1.2/ICRS-TT-WAVELENGTH-TOPO.xml"/>
        <crd:AstroCoords coord_system_id="ICRS-TT-WAVELENGTH-TOPO">
            <crd:Position2D unit="deg">
                <crd:Name>RA,Dec</crd:Name>
                <crd:Value2>148.88821 69.06529</crd:Value2>
            </crd:Position2D>
        </crd:AstroCoords>
        <AstroCoordArea coord_system_id="ICRS-TT-WAVELENGTH-TOPO" ID="Area"/>
    </ObservationLocation>
</ObsDataLocation>