arith.diff, arith.ratio

[Markus Demleitner]

Hi,

On Wed, Sep 01, 2021 at 07:05:18PM +0200, Mireille LOUYS wrote:
> we did discuss this point some years ago.
> we reached the agreement that when a mathematical operator is applied to
> physical quantities,
> what is important is still to keep the emphasis on the physical quantity and
> not on the operator applied to it.

Hm... what you're saying is that in data discovery, if I'm looking
for columns matching pos.eq.ra;%, I want RA-like things, and
differences with other RAs count?  Yes, that's a fair point.  And
conversely, I really don't see a plausible case for people wanting to
discover "columns that are differences".

On the other hand, if I'm a client trying to figure out what to do
with a table, and I'm looking for columns with UCDs matching
pos\.eq\.ra.* in an attempt to find columns suitable for use in
positions -- yes, I admit we should have better ways to do that, but
so far we don't --, using a pos.eq.ra;arith.diff column quite
certainly is wrong (unless it's a difference to a constant RA, of
course).  For arith.ratio, inspecting the unit would probably prevent
bad mistakes, but for arith.diff, even the unit would look right.

I'm not sure what to make of that observation; I guess for the
record, I'd say that there's sometimes a certain tension between the
UCD use cases of data discovery and aiding machine interpretation of
tables.

          -- Markus