Defaults in STC

[Anita Richards]

STC states that there are no global defaults, which is reasonable if
this means that the schema requires every element to have a value.  In
general data providers should be encouraged to describe their data
rigorously. However for some data it may not be practical for a number
of reasons, possibly because the data provider thinks something is
irrelevant and is too busy, or because some details of the data source
are lost in the mists of time.  Hence we might end up rejecting highly
desirable data, for example a great deal of spectral data such as HI
data does not say what reference frame or Doppler conventions it has
used and in many cases the spectral or velocity uncertainties are
greater than the differences between LSR, Heliocentric, and between
optical, radio or relativistic.

There are several solutions and these could be applied as local
defaults ('local' meaning either the data provider or the local VO
data centre etc.).  In the above instance allowing 'Not Available' as
a reference position location might work, but might break conversions
between e.g. wavelength and velocity.  Alternatively someone with
knowledge of the probable observing procedure used might say 'call it
all heliocentric, optical'.  Bob Hanisch proposed that metadata
accuracy should have a flag which could reflect this - or the data
provider could increase all relevant uncertainties by an appropriate
amount.

Section 2 of STCv1.0 (p. 4) suggests that such context-dependent
defaults might be acceptable. Section 6.2.4 describes how
observational data uses two STC objects; the observatory data
(location etc.) is used for some operations on observational data.
Section 7.1 says that all elements can contain actual values or refer
to other elements. Is it possible to use these sructures to supply
defaults where these involve making educated guesses about the
observattion procedure, or even other elements? In some cases
(e.g. antenna positions) there can also be legitimate improvements
when more accurate data becomes available.  If not, local defaults
could simply be hardwired.


Summary:

Encourage complete descriptions but rather than reject data:
Allow Not Available
Explain how to use references/local defaults
Use metadata accuracy flag system to warn when this has been done

thanks

Anita

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Dr. Anita M. S. Richards, AVO Astronomer
MERLIN/VLBI National Facility, University of Manchester,
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).