Re: [galeon] How to map a parameterized projection to an EPSG code

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Hi Jon, all,

Jon Blower wrote:
Hi John,

This is a serious general problem I think.  In the case of
parameterized projections you could express the projection in
something GIS-friendly like WKT, but not all of these projections will
have an EPSG code of course.  (CRS identifiers don't need to be EPSG
codes, one could come up with another authority, but that's not the
real issue.)

It is a real problem that there are an infinite number of CRSs but WxS
expects the CRS to be identified by a code.  Perhaps a WxS extension
could use a resolvable URI as the code, which points to a document
that describes the CRS in machine-readable form?

The WCS 1.1 spec says that WCS messages can reference or contain CRS definitions. Most of the locations in the XML Schemas seem to only deal with referencing CRS (with URI) but I may just have missed the places where it can contain them. The specs discuss two ways of referencing a CRS: 1) with a URL that can be resolved to a definition of some sort; and 2) with a OGC CRS URNs (which can get quite ugly and unruly esp. for parameterized projections). As far as how the CRS are defined, it seems to be either OGC CRS URNs (which can either identify or define a CRS) or an XML document containing a GML CRS definition.

I think this *ability* is more wide spread than WCS as the specification also mentions OWS Commons which I believe is used more widely than WCS. However, I don't think the *use* of general CRS definitions is widely used or tested.

We also have the problem of the "arbitrary" coordinate systems that
can only be expressed through an exhaustive listing of the lat-lon
points that comprise the grid.

I'm not sure of the current status but there is a change request to add the ISO 19123 CV_ReferenceableGrid to GML. Which would then hopefully percolate down into WCS and other WxS. The referenceable grid allows for just this kind of "arbitrary" grid.

Ethan




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