Characterisation draft

[Anita Richards]

Below are suggestions for levels of compliance (mandatory keywords)
within the Characterisation model, discussed with Mireille and
Francois. Further comments please - I hope to produce a formatted
section for the main Characterisation document

http://alinda.u-strasbg.fr/Model/Characterisation/CharacterisationDraftMai06/

by Moscow so any comments received in the next few days will be
considered, otherwise they will be used for a subsequent draft.

I will send some additional thoughts in a separate email,

Thanks,

Anita

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Unless otherwise stated I make no assumptions about what coordinates
any axis, region, etc. applies to


Minimum requirements:

+ Reuse of STC definitions wherever possible
+ Consistency with the Registry DM in areas of overlap and with other
   Obs DM when these develop.
+ Consistency with DAL-oriented models e.g. SIAP, Spectral DM, STAP
   (time) and with the eventual Catalogue model, in areas of overlap.

These models allow a query to return (lists of) data which does not
explicitly exclude a region of interest. One of the goals of Char is
to allow more refined selections without disabling very simple queries
e.g. related to VOEvent.

In some places we mention the use of defaults for values not provided;
we do not think that these will be coded into the description, but
software which looks for a value of  e.g. Support and does not find
it, should know to look for Bounds instead.

The unit and coordsystem 'must' be taken from a controlled vocabulary
(STC?).

The first three levels on each axis (Location, Bounds, Support) take
explicit numerical values (or arrays of values) in order to be
accessible to any tool.
\footnote{At present all compulsory elements and any other elements
which need to be widely accessible should be given directly (not
functions nor URIs) since few (if any) software tools can cope with
indirect references.  The more detailed levels may be direct values or
indirect references to functions or URIs (e.g. for a look-up table or
weight map) but the data providers should be aware that the cost of
extra precision may be a delay before full use is made of the
information, e.g. if an aperture-matching tool becomes available it
may be able to use the description of resolution by major and minor
axes and a position angle in Support, but not by a Gaussian PSF
function - hence you are strongly encouraged not to omit optional
coarser levels of description if you intend to provide finer levels.}



COVERAGE

A description should provide at least one Coverage axis ("Axis
Frame", "Axis").

The unit and coordsystem 'must' be given for each Axis present (these
may be relative to an internal reference only, e.g. (x,y) spatial
coordinates. In such a case the Location and Bounds 'must' be given on
that axis).

The value 'must' be given for either the Location or the Bounds on
each Axis.
Values 'should' be given for both Location and Bounds.

If Location is not given then for some Axes the default Location can
be the mid-point of Bounds\footnote{in some cases this might be 
complicated
(e.g. some spatial coordinates) or impossible}.

If Bounds are not given then in some cases defaults are possible
e.g. if a spatial axis has Coordsys ICRF the default would be all
sky\footnote{ a more restricted coverage might be derived once there
is a link to Observation and the telescope location.}

All three of the Space, Time and Spectral Coverage Axes 'should' be
given\footnote{some might be considered irrelevant for simulated data, or 
not
conventionally provided e.g. for old spectra with no time stamp}.

The Observable Axis 'should' be given\footnote{its omission may seem
reasonable if publishing e.g. the coverage intended for a future
survey}.

Other Coverage Axes e.g. Velocity 'may' be given.

Support 'should' be given, otherwise it defaults to Bounds, if present.

Sensitivity 'may' be given (e.g. the URI of a weight map, or a
function)\footnote{Here, Sensitivity is the dependence of a detector
response or equivalent with position on the given axis. This is not
the limiting sensitivity in the sense of the faintest detectable flux,
which is given by the lower Bound of the Observable axis}.

Separate Units and CoordSystem 'may' be given for each of these coverage
layers, if not they will default to the units and CoordSystem used for
the Axis Frame (i.e. when the axis was first defined).

FillFactor 'should' be given on any axis where the actual coverage is
significantly less than 1 (i.e. 100%) but the filling is too complex
in too many irregular regions to be described practically using
Support or Sampling. FillFactor applies to the usable fraction of
data within the whole Bounds.

FillFactor 'may' be given in addition to Support and/or
SamplingPrecison in which case:
If SamplingPrecison is not given but Support is given
FillFactor =/< Support/Bounds

If Support is not given but SamplingPeriod and SampleExtent are given
FillFactor = SampleExtent/SamplingPeriod
If Support, SamplingPeriod and SampleExtent are given
FillFactor = Support/Bounds *  SampleExtent/SamplingPeriod

and the data provider should take care that the values and units given
are consistent with these relationships.

OTHER AXES

These relate to a specific COVERAGE Axis Frame but themselves contain
Location (e.g. typical or reference value), Bounds etc.

If there are many areas of Support within the coverage, the Resolution
and Sampling Precision refer to the inside of each Support area and they
are assumed to be consistent within each Support area (if not, each area
must be described separately).

Resolution and SamplingPrecision 'may' be present and if so:

You 'must' give the unit and Coordsys on axes where the units of the
Axis Frame would not make sense or are ambiguous (e.g. sexagessimal)
otherwise the unit default is that of the Axis Frame.

You 'may' give the the unit and Coordsys for any Resolution or Sampling
level.

If Resolution is present:
You 'must' give resolutionRefVal (i.e. Location)
You 'should' give resolutionBounds (default is resolutionRefVal)

You 'may' give resolutionSupport\footnote{
This has alternative forms for some axes e.g. spatial 2D (see STC):
       box size (x, y)
       or vectors ([x1, x2], [y1, y2])
       or ellipse (MAJ, MIN, PA)
and you 'must' give the appropriate number of values if you give any.}

You 'may' give resolutionVariability (as a function of value on that 
axis).

If SamplingPrecision is present:

You 'must' give a samplingPrecisionRefVal (i.e. Location) for the
samplingPeriod.
You 'should' give a samplingPrecisionRefVal for the sampleExtent.

You 'should' give Bounds for the samplingPeriod and the sampleExtent
(if not, these default to the relevant samplingPrecisionRefVal
values).

You 'may' give Support  for the samplingPeriod and the sampleExtent
(if not, these default to the relevant samplingPrecisionBounds).

You 'may' give samplingPrecisionVariability (i.e. Sensitivity) in the
form of a samplingPrecisionMap to describe variations along an axis.

ACCURACY

Each Axis Frame 'should' have associated Accuracy values for the
precision of measurements along that axis, divided into statistical
and systematic uncertainties. For each measurable quantity:

You 'must' give the  unit and Coordsys on axes where the units of the
Axis Frame would not make sense or are ambiguous (e.g. sexagessimal)
otherwise the unit default is that of the Axis Frame.

You 'may' give the unit and Coordsys for any accuracy axis\footnote{for
example normalised units such as a flux accuracy of 0.03 (3%) of any
given flux measurement.}

You 'should' give the ErrorRefVal (typical value i.e. Location)
You 'may' give the ErrorBounds for uncertainties which vary along the
domain of the axis
You 'may' give an ErrorMap (as a URI) to describe the variation of
errors with location.

FLAGS

For each Axis Frame (spatial, spectral, observable (e.g. flux) etc.):

There 'should' be a flag to indicate if it is an independent or a
dependent variable.

There 'should' be a flag to indicate its calibration status (default
UNCALIBRATED, so it is in your interests to provide a better status if
applicable if you want your data used!)

There 'may' be a flag to indicate Sampling Status (this 'should' be
provided where it is customarily relevant).




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Dr. Anita M. S. Richards, AstroGrid 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).