[PROJ] CRS aliases

[Martin Desruisseaux]

Hello

Le 21/07/2023 à 15:34, Even Rouault a écrit :

> If I remember well, this part of the API was something I coded early 
> in the PROJ 6 work, taking inspiration (and strugling to understand) 
> from GeoAPI classes. Various classes GenericName, LocalName, 
> NameFactory etc to deal with basically a string. So the API is there, 
> there's minimum testing for it in test/unit/test_common.cpp, but it 
> hasn't been really used for useful stuff.

The complexity come from the fact that aliases are defined in ISO 19111 
as instances of GenericName, which is a class defined a bit succinctly 
in ISO 19103. The main idea is that they are like file paths. For 
example using ":" as a separator, a generic name could be 
"IOGP:EPSG:SomeAlias", meaning that "SomeAlias" is a name defined in the 
"EPSG" scope, which is itself defined in the "IOGP" scope. This is only 
an example, actually I found few examples of scoped names (including in 
EPSG database), so it is difficult to said what the established practice 
is. Added to the fact that ISO 19103 makes a distinction between "scope" 
and "namespace" but I found few guidance about what the distinction 
should be, it makes GenericName a bit open to interpretation.

If desired, PROJ could use LocalName (a specialization of GenericName 
containing only the "SomeAlias" tip of above example) and forget the 
other classes for now. If PROJ is willing to do a little bit more 
advanced implementation, they could declare "EPSG" (or other relevant 
authority) as the scope of that LocalName.

Note: above discussion should not be confused with URN in OGC name 
space, for example "urn:ogc:def:crs:EPSG::4326". Despite their 
similarity, they are different objects in ISO models: GenericName for 
the former, versus Identifier for the latter.

     Martin