Describing things for software to read

[Doug Mink]

Martin Hill wrote:
> 
> The data modelling team are working on describing data values, which is
> useful for data servers.  This forum probably needs to look at (if it's not
> done already!) how it will describe metadata in the registry using terms that
> software can interpret.  How, for example, will we say that a particular data
> set contains intensities in absolute ergs, while another contains the same
> information but in magnitudes?  Similarly with passband filters, pixel
> resolutions, data quality, etc, etc.
> 

We very quickly get into semantics here.  Obviously UNITS has to be an
attribute of any measurement.  Bandpasses get lots trickier because filter
passbands have a shape and not just limits or a simple defining equation.
In some cases, you need to know that a defined filter is used; in others,
knowing the upper and lower wavelength/energy/wavenumber limits (UNITS
again) is enough to decide whether the data is useful.  In still other
cases, knowing the wavelength category--gamma ray, xray, UV, visible,
IR, sub-mm, radio, or subdivisions thereof--might be enough.  Software
which accesses the archive needs to be able to translate units at some
level; I don't think it would be a good idea politically to require all
optical data to be translated into photon energy to enter the VO when it
starts out in wavelength, or vice versa for x-ray data.

-Doug Mink
 Telescope Data Center
 Harvard-Smithsonian Center for Astrophysics