Coverage subgroup update

Hi all

Sorry for being a bit quiet on this over the last month or so - it was as a
result of a combination of holiday and other commitments.

However, some work on the topic has been continuing.  Here is an update for
discussion in the SDW plenary call tomorrow.

In particular I had a meeting in Reading on 5 July with Jon Blower and
fellow-editor Maik Riechert.

During that we came up with a proposed approach that I would like to put to
the group.  The essence of this is that we take the CoverageJSON
specification of Maik and Jon and put it forward as a potential W3C/OGC
recommendation.  See
https://github.com/covjson/specification/blob/master/spec.md for the
current status of the CoverageJSON specification.

That spec is still work in progress and we identified a couple of areas
where we know we'll want to add to it, notably around a URI convention for
identifying an extract of a gridded coverage, including the ability to
identify a single point within a coverage. (Some initial discussion of this
issue at https://github.com/covjson/specification/issues/66).

Maik and Jon understandably feel that it is for others to judge whether
their work is an appropriate solution to the requirements of the SDW
group.  My opinion from our discussions and initial review of our
requirements is that it is indeed a good solution and I hope I can be
reasonably objective about that.

My intention is to work through the requirements from the UCR again and
systematically test and cross-reference them to parts of the CovJSON spec.
I've set up a wiki page for that:
https://www.w3.org/2015/spatial/wiki/Cross_reference_of_UCR_to_CovJSON_spec
 That should give us a focus for identifying and discussing issues around
the details of the spec and provide evidence of the suitability of the
approach (or not, as the case may be).

There has also been substantial interest and work within the coverage
sub-group on how to apply the RDF Data Cube vocabulary to coverage data,
and some experiments on possible adaptations to it.  The main potential
drawback of the RDF Data Cube approach in this context is its verbosity for
large coverages.  My feeling is that the standard RDF Data Cube approach
could be a good option in the subset of applications where the total data
volume is not excessive - creating a qb:Observation and associated triples
for each data point in a coverage.  I'd like to see us prepare a note of
some sort to explain how that would work.  I also think it would be
possible and desirable to document a transformation algorithm or process
for converting CoverageJSON (with its 'abbreviated' approach to defining
the domain of a coverage) to an RDF Data Cube representation.

So the proposed outputs of the group would then be:

1) the specification of the CoverageJSON format, to become a W3
Recommendation (and OGC equivalent)
2) a Primer document to help people understand how to get started with it.
 (Noting that Maik has already prepared some learning material at
https://covjson.gitbooks.io/cookbook/content/)
3) contributions to the SDW BP relating to coverage data, to explain how
CovJSON would be applied in relevant applications
4) a note on how RDF Data Cube can be used for coverages and a process for
converting CovJSON to RDF Data Cube

Naturally I expect to discuss this proposal in plenary and coverage
sub-group calls!

Best regards

Bill