An STC "when" and "where" example

[Rob Seaman]

Roy Williams writes:

> 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.

Note that section 4.6 of STC v1.21 makes it clear that Time and 
Position objects can be included separately from Velocity, Redshift or 
Spectrum objects.  We should be able to include only what is needed.

> <?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>

Note that the real complaints are with the verbosity of XML.  Here is 
Arnold's original example as an STC-S string expression:

>> ObsDataLocation
>> ObservatoryLocation
>> Position GEO_D TOPOCENTER SPHER3 248.4056 31.9586 2158
>> ObservationLocation
>> Time UTC TOPOCENTER 2005-04-15T23:58:55 Error 5
>> Position ICRS TOPOCENTER 148.88821 69.06529 Error 1.0 1.0
>> Spectral TOPOCENTER 460 unit nm Resolution 40

This is fewer characters than your example with a much greater 
informational content.

Bottom line - either the information is necessary or it isn't.  If all 
these details are necessary (and I'll assert that they are for many 
purposes) then the information must be conveyed in some fashion.  
Certainly the ObservatoryLocation could be conveyed with a separate DB 
- Kitt Peak ain't going anywhere.  Alternately, the VOEvent might 
convey a barycentric value, not topocentric.  I suspect that VOEvent 
will benefit from the flexibility of allowing either strategy.

> -- Wavelength information, please can you explain why this is in with 
> the space-time coordinate systems?

Perhaps we're just viewing the situation too closely.  We aren't really 
seeking a standard way to convey coordinates and times.  VOEvent is 
seeking precisely what STC is selling:  a standard way to convey 
"Space-Time (and relate coordinate axes) metadata".  Arnold and his 
collaborators view velocity/redshift as a key element of a coordinate 
specification and therefore spectral information also.  Again, these 
are optional, although I haven't fully digested the semantics.

Bottom line - what is a VOEvent without a spectral characterization?  
Perhaps STC can provide this (or ancillary access to this) as well as 
the spatial/temporal coordinates.

Rob Seaman
NOAO