# Spectrum data model: accuracy

Anita Richards amsr at jb.man.ac.uk
Fri Sep 15 04:42:42 PDT 2006

In reply to Alberto and in praise of UNKNOWN:

>> Location.Value =  1975 Jan 1
>> Bounds.Extent = 10 years
>
> This is very confusing.
> If the date is not know very precisely, the accuracy must be not zero.
> The accuracy, in this sense, is linked to the concept of "how
> well I know this measurement", hence it is linked to the Error.
>
> The bounds are not errors. The bounds tell the starting point and the
> ending point (and each with its own error) of a property of an observations,
> and have nothing to do with the precision with which we know the location.
>
> Bounds.Extent = 10 years means that the observation spanned across 10 years,
> and the user will think that the spectrum is a co-addition or anyway an
> assembly
> of spectra taken within 10 years.

Actually the intended interpretation of Bounds is not that data were taken
continuously within the Bounds, but that the data occur somewhere inside
the bounds.  I sympathise that it could be confusing, but in Char we
decided not to have errors on Bounds; the Bounds are themselves the outer
limits to all errors, in a sense.

> ---------------
>
> To go back to the required/optional fields:
> the initial paragraph in 4.6 "Accuracy Fields"
> states that all the Accuracy fields are optional.
>
> I think this is simply bad. A data provider for sure
> does know the accuracy, at least at a course level,
> and if s/he doesn't, s/he should admit it, frankly, not silently.
>
> An example (cfr example above):
> What could happen if an astronomer downloads the spectrum taken 1-Jan-1975
> without knowing that the uncertainty on the date is 10 years?
> the answer is: wrong science.
>
> Physics requires measurement errors, the VO MUST provide them.
> And if it is impossible to know the errors, the VO MUST state that.
> Error = UNKNOWN is much better than silence.

That is fine, I would be happy with your previous email about more
stringent compulsion if 'UNKNOWN' or 'N/A' were allowed for 'must' fields.
I repeat what I ahve said before, some useful data is taken from
publications - typically stored on a university server for non-tabular
data - but all details were not recorded and the person responsible has
vanished... Or, there are areas of astronomy/theory where some details are
felt to be irrelevant.  I am not sure about branching, I fear that might
cause more confusion than it solves, if we end up with too many twigs.

I agree that silence can be misleading and what I am worried about is that
in fact people do something worse - if we are too rigid then what happens
in the Registry is that people put in the answer to the question they
think we should ahve asked - e.g. if we say 'must be in degrees' people
thnk that they should be allowed to put in arcseconds as an accepted
astronomical unit - and they do... or if we aks for total error and they
only ahve a statistical error, they will put that in anyway.... 'UNKNOWN'
is indeed preferable.

all the best

Anita