[COVERAGE]

[David Berry]

Pat,
    I must not have expressed myself clearly - I am all in favour of
having uniform models for things like COVERAGE. My suggestion was that
each domain (i.e. a subset of the axes, 2 for spatial, 1 for
spectral/temporal) could be described uniformly using components
representing sensitivity, resolution, sample size, and error. This seems
pretty similar to the CVO model you described - your "bounds" is my
"sensitivity", and I've not explicitly sectioned off the other items into
a separate "sampling" structure. The only substantive difference that I
can see is that I've got "error" and you have got "fill factor". I've
not problems with fill factor being added to the list.

David



----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

STARLINK project                        Centre for Astrophysics
(http://www.starlink.ac.uk/)            University of Central Lancashire
Rutherford Appleton Laboratory          PRESTON
DIDCOT                                  United Kingdom
United Kingdom                          PR1 2HE
OX11 0QX



On Fri, 31 Oct 2003, Patrick Dowler wrote:
>
> Coverage, or "bounds" as we call it in CVO, is a 1st order summary of the
> full blown description. However, the model here doesn't make sense to me
> because it does treat things uniformly when they are uniform and treat
> them differently when they are different.
>
> The WCS is typically ~2d, so the spatial coverage/bounds has to be a 2d
> construct. CVO uses polygons, but other  shapes would be fine depending on how
> exact one wants to be... the idea is that you want to tell the difference
> between "in" and "out" and whether two things overlap/intersect.
>
> Spectral and temporal axes are 1d, and a 1-d bound is an interval (loValue and
> hiValue below). They are not error values on the refValue. What is "time of
> observation"? The  observation starts at loValue and ends at hiValue, no??
>
> In the CVO model, each axis (spatial, spectral, temporal) has a  "bounds"
> object and a "sampling" object. The bounds are polygon or interval
> (currently). The sampling object has several parts: number of bins, bin size,
> resolution, and fill factor. One can compute the Nyquist ratio from bin size
> and resolution, which is what Alberto was referring to about data being
> undersampled). So, all axes have the same sampling description and the bounds
> description dpeends on the dimensionality. If you split up the two spatial
> axes in an attempt to have 4 x 1d axes, you could use intervals for bounds
> everywhere, but then you are essentially putting an axis-aligned bounding box
> around the polygon, which is a worse characterisation of the WCS, for no good
> reason.
>
> My thoughts, based on our experience actually trying to model this in general,
> put it all into a database, actually describe different types of data this
> way (WFPC2 images, 2QZ spectra, ROSAT fields, CFHT 12K images, etc)
> and then query it in a uniform and general fashion.  Yeah, there are things
> I'd do differently, but not much differently :-)