DataLink local_semantics optional column proposal

[Markus Demleitner]

Hi DAL,

On Fri, Aug 12, 2022 at 09:54:27AM +0100, Mark Taylor wrote:
> So I'd like to see an additional column to facilitate this.
> Markus has suggested the following definition for such a column:
> 
>    column name: local_semantics
>    type: text
>    UCD: meta.id.assoc
>    description: An identifier that allows clients to associate rows from
>       different datalink documents on the same service with each other.
> 
[...]
> 
> Any comments?  Assuming some agreement or lack of disagreement is
> established here about the general idea and specifics, then if
> at least one data provider implements this I will add code in
> topcat to make use of it.

I think it's a good thing to have that, and in my book it's fairly
orthogonal to semantics (or any other feature we have in datalink so
far).  Hence, I've put it into DaCHS (SVN only so far; DaCHS
operators: if you want this, let me know and I'll make a beta
release), and I've taught a service where this looks reasonable to
spit out local_semantics.

That's PPAKM31, a service giving narrow-band maps of HII regions in
M31 from cubes (reference URL:
https://dc.zah.uni-heidelberg.de/browse/ppakm31/q); the datalink
essentially links together the maps in the various bands extracted
from each cube, and hence the local semantics is just an (opaque)
label with the band info.

To try this, go to the TAP service at <http://dc.g-vo.org/tap> and
execute:

  select accref, imagetitle, cube_id  from ppakm31.maps

In TOPCAT, you can then configure an activation action "Invoke
Service"; the default "View Datalink Table" is fine.  When you then
activate a row, you will see the local_semantics column already in
non-aware TOPCATs.  Mark's proposed functionality would then let
people say "whatever band I'm actually looking at, I always want the
[OIII]5007 image of the field in my extra datalink window".

Which, I'd say, is a highly reasonable thing to want.

So... you'd totally have my support for adding this to a
Datalink-1_2-Next page on the Wiki.

          -- Markus