Who chooses? (was Re: content, format, ctype, or xtype ?)

[Rob Seaman]

I'm snow-blind with the blizzard of messages today.  Returning to  
Mark's use case from yesterday...

On May 11, 2009, at 5:48 AM, Mark Taylor wrote:

> The use case which I have in mind (and I think Doug is thinking along
> similar lines) is this: a user acquires a VOTable from some source -
> perhaps TAP, perhaps not.  It contains a column X whose contents
> is a string in iso-8601 format - this is perhaps identified by
> utype with part of the STC data model, or with some other data model,
> or perhaps is not.  The user loads the table into TOPCAT
> (or some other generic table handling software) and wants to make a
> plot with column X as one of the axes.
>
> As far as TOPCAT can tell, the column contains a string, and so it is
> unable to make a plot with it, or otherwise do anything much apart
> from display the string contents.  If it understood that the column
> contained a string with the semantics of an iso-8601 date/time,
> it could make this plot.  Yes it may be possible to glean this
> information by inspecting the utype, but in order to do that it needs
> to have an understanding of the data model in question - a lot of work
> for the developer, and needs to be updated every time a new data model
> appears or is modified.  Moreover, the additional, probably rather
> detailed, information supplied by the utype is not relevant for this
> kind of processing.
>
> You can think of similar stories for 'ctype' (or whatever) values of
> stc-s, stc-x, sexagesimal, and other possibilities of your own device,
> including domain-specific ones.  It should not be necessary to invent
> a data model in order to flag this kind of thing, partly for practical
> reasons (you need to reach agreement about a data model and update
> software each time), and partly because use of a data model is  
> orthogonal
> to this issue.

...and subtracting out all the high-falutin' computer science issues  
from today, we see that this is simply a question of whether to flag  
some value.  Whether the value is flagged or not, if TOPCAT is to do  
what Mark's user's want, then TOPCAT has to be able to parse ISO-8601  
datetime strings or sexagesimal strings or stc-s strings.  These  
parsing methods must all be in place, the question is how to trigger  
them and who decides when to do so.

Requiring an explicit metadata flag (whether expressed as a UCD,  
utype, ctype, xtype, unit or whatever) implies that the data provider  
(or her minion programmers) should be the one selecting how an  
application like TOPCAT chooses to interpret different values.  This,  
I think, is the real underlying issue.  Rather, might it not be  
asserted that TOPCAT is a power tool belonging to the user?

With a method to parse sexagesimal values - a method that is required  
in any event - isn't it trivial for TOPCAT to activate user controlled  
plotting capabilities for such string valued columns?

Rob