STC and Characterisation ranges

[Anita Richards]

Ranges in data specifications - Observation/Characterisation and STC:

At present many elements are allowed to have one or no values but
others can have a range of values.  I would like to see more things
take ranges unless there is some problem I have overlooked.  It will
add a little complexity but one way round this would be to make max
and min an optional sub-class in addition to central value and
uncertainty.

For some things like the coordinate system or the UCD for an element
this obviously has to be single-valued if any value is given.

However in other cases it is not clear why some elements are
restricted to being single-values and others can have sub-elements or
multiple values;

e.g. the wavelength range can have max and min but the pixel size is
fixed.

However, for example the exact angular resolution and pixel size of an
image extracted on demand from visibility data is not known until the
request is made, and maybe I have just come across peculiar x-ray data
but in quite a few examples I have used the pixel size varies within a
single
image (smaller pixels in low signal-to-noise areas are binned in 4's,
16's etc. to improve signal-to-noise). This is particularly important
for e.g. virtual images from visibility data where the user needs to
select the desired resolution or pixel size (e.g. the NVSS cutout server
http://www.cv.nrao.edu/nvss/postage.shtml )

It would be useful to be allowed to have a max and min here, also.
This cannot be done using uncertainties since we do know exactly how
large the pixel size(s) are in any one real image, but either the
image is virtual until requested or is varied.  There is a separate
situation where one might not know the pixel size in arcsec to high
precision e.g. in an old digitised image with serious edged
distortion; here uncertainty is appropriate.  Analogously, the uncertainty
on the centre of a wavelength interval is not the same as the width of
that interval.

Hope this makes sense

If no-one agrees maybe someone can suggest an alternative means to
describe the sort of data with varaible pixel size ec. as described above
in order to allow users to access the data to their exact specifications
(as in cut-out servers which allow resampling for example and other
virtual data).

Looking forward to using the models!

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).