utype for STC region in SIAP query response

[Norman Gray]

Arnold and all, hello.

On 2011 Dec 16, at 15:18, Arnold Rots wrote:

> Yes, Alberto is correct, it should be:
>     stc:ObservationLocation.AstroCoordArea.Region

Righto!

> The remaining question might be whether one needs to identify a
> particular instance of a Region as a footprint. I would argue that, in
> a way, every region is a footprint of some kind. If it is a member of
> an ObservationLocation, it is the footprint of an observation; if it
> is a member of a CatalogEntryLocation, it is a catalog footprint.
> One might reinforce the point with the FIELD's name, but that does not
> change the nature of the item as defined by the utype.

Seen from the point of view of geometry on the sky, yes, a footprint is just a Region.

However one might want to produce a VOtable in which one includes both the region of sky which was in the field of view of a satellite during one orbit, and the region of sky in which the satellite actually took data -- they might be different for any number of reasons.  From the point of view of STC, these are both just Regions.  From the point of view of the client, however, these are very different things, and it would be natural and helpful to this client if the data were described in a data model which distinguished the notion of 'footprint' (meaning the area of sky where there are observations) from whatever term you'd use for the other region.

This is the point: an application that plots sky regions has a natural data model which consists of only Regions (something like STC); an application which is searching for datasets which cover a particular point on the sky has a _different_ natural data model (something more like Char).  Neither is _the_ correct dataset, but the data can easily conform to both.

Best wishes,

Norman


-- 
Norman Gray  :  http://nxg.me.uk
SUPA School of Physics and Astronomy, University of Glasgow, UK