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Hi all, [cc'd to wcs.rwg as well]This is a great discussion (even as it has wandered in & out of several mail-lists and subject headings; if someone wants to forward this to the wfs.rwg that'd be great. :-).
Ideally (in WCS 3.0 ??), WCS would be agnostic about the underlying data structure. It would describe a "pure" spatial function, and (as for formats and grid sizes today) it would offer to represent that function using the user's or providers' choice of data structure: a rectangular grid perhaps, or contour lines, a TIN, an irregular point mesh, hexagonal tesselation, etc. (The provider might highlight some of these as more "natural" / "native" / "raw" than others.)
Any of these data structures could obviously be represented as a collection of atomic features with a common schema; or as a single feature with space-varying attribute(s); or as observations. Coverages are a matter of interpretation; the provider can suggest a "coverage interpretation" of the provided data by supplying an interpolation method alongside the data; but the user might ignore this and treat each "datapoint" in isolation. So WFS / WCS / SOS could provide different views (or suggest different interpretations) of the same underlying information.
In fact, one of the key reasons for limiting WCS to grids so far has been interpolation -- needed whenever a client request entails reexpressing the coverage using a non-native geometry (e.g., in less spatial detail or in a different CRS); or requires values at the edges of coverages or of bounding boxes. For WCS 1.0 and 1.1, the costs of agreeing on a more general (non-grid) treatment of interpolation outweighed the benefits; but it would be great to do this at some point.
Simon.Cox@xxxxxxxx wrote:
/Notwithstanding/ the current interest in fully unifying the feature and coverage views(through the CRS generalization activity, for which the logical outcome is CRS=FeatureType),I believe that feature, coverage, observation, catalogue can /already/ be seen as merely different views onto, projections of, or sections through, the underlying data soup.There now appears to be some agreement that a "feature" may have a property whose value varies in some way "across" the feature, for example in time or space (see sub-clause 6.5.3 and Figure 4 of Observations and Measurements- OGC 05-087r4 http://portal.opengeospatial.org/files/?artifact_id=17038) and that this value is a "Coverage" whose domain extent is the feature.In the proposed update to the SamplingFeatures clause (see OGC 07-002r1 http://portal.opengeospatial.org/files/?artifact_id=20934&version=1 <http://portal.opengeospatial.org/files/?artifact_id=20934&version=1>)this is generalized further to allow variation with respect to non-spatial axes inherent to the "feature" (see sub-clause 7.1 and Figure 2).I've also been thinking about the implications of this model in terms of service composition:For example, if a feature type has a property with a coverage value, then a WFS "GetFeature" request for such a feature might use a GetCoverage request to a WCS "cascaded" behind the WFS in order to fully compose the response. There are some other similar interactions potentially implied by other SOS and specialised WFS operations.I had presented this in the form of what George Percivall calls "your horrible powerpoint picture"a couple of times in OGC forums mid last year (e.g. in the SWE WG at the Edinburgh TC).I think George's main problem was that my "SOS" pattern put a WFS and WCS behind the SOS,instead of vice-versa, which would match the idea of "observations" as being in some sense "more primitive" that features. However, I still stand by that analysis, and I have now added a couple of other variants, based on the "Sampling Feature Service" viewpoint, the "Domain Feature Service" viewpoint, and the "just the data" viewpoint.They are still in horrible PPT-ML, pending Bryan helping me figure out how to show it in UML,and definitely could use some refinement, but maybe time to share the ideas ... see attached.Simon
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