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I don't see how a URN can define a CRS - URN's are to be identifiers of resources and should resolve to the definition of a CRS. I think from this discussion it seems that there is a need to register a "CRS with parameters" (somewhat like a function signature) and then define a grammar by which this CRS' parameters are passed. R
-----Original Message----- From: galeon-bounces@xxxxxxxxxxxxxxxx [mailto:galeon-bounces@xxxxxxxxxxxxxxxx] On Behalf Of Ethan Davis Sent: October 9, 2008 10:42 AM To: Unidata GALEON Subject: Re: [galeon] How to map a parameterized projection to an EPSG code 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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