[MetaCRS] Re: Introduction]]

[Jody Garnett]

Frank Warmerdam wrote:
> Jody Garnett wrote:
>> Having access to shared dictionaries would be a good thing; and 
>> something we could easily slot into GeoTools (while we may not be 
>> interested in EPSG since we use that database directly).
>>
>> Dictionaries that would be worth sharing:
>> - Authorities like AUTO and AUTO2 from the WMS specification
> Jody,
>
> The AUTO WMS projections are a bit more involved than a simple 
> dictionary, as far as I understand because many of these are
> parametric. That is the definition can include additional parameters 
> for stuff like central meridian.  This means it requires more than
> just a dictionary.  Instead I think it essentially requires code to 
> valuate the definition and transform it.
Understood; I implemented them for GeoTools. They are however defined in 
a "template" fashion; so if the dictionary definition supported some 
substitutions we should be fine.
>> - Additional informal EPSG codes
> There is no such thing as an informal EPSG code.  What I believe you 
> mean is additional CRSes informally wedged into the EPSG
> space by assigning ids that will hopefully never conflict with 
> formally assigned ids.
That is exactly what I mean; we have a list of them in geotools; and it 
would be a good thing to share would it not?
>> Frank right now we have an "alias" dictionary that we use when 
>> reading WKT from a range of sources in order to make it conform to 
>> the standard, it would be a good candidate to share since it is 
>> exactly the kind of information that is a pain to build from real 
>> world experience.
> Yes, absolutely.  In OGRSpatialReference-speak I think you are 
> referring to what I call a "morpher".  Basically something that
> transforms a flavor of WKT into what I consider standard WKT (but 
> which is more accurately just OGR WKT).  I maintain a morpher for
> ESRI PE WKT that is pretty decent.  I am also looking at developing 
> something similar for various versions of Oracle WKT.
That is exactly what I am talking about; I would be interested in 
knowing where you have improved on WKT; I am not familiar with anything 
other then the "well-known" flavour of WKT; generally treating WKT from 
shapefile PRJ files and from PostGIS spatial-sys table with the 
"morpher" stick...
> I don't believe that ReferencedIdentifier (aka Authority) appears in 
> the EPSG tables since everything is implicitly the EPSG authority.
> That's why I say I don't think it would be *exactly* the EPSG tables.  
> We need in some fashion to differentiate. I'd add that
> many CRS dictionaries use non-numeric identifiers.
True.
> Well, there is the GML3 specification of course, and a particular
> section on GML CRSes.  The new(ish) EPSG registry returns GML
> definitions as I understand, though I haven't reviewed it recently.
>
> http://www.epsg-registry.org/
Thanks those examples helped.
> A dictionary would presumably just be a collection of these, though 
> likely normalized to refer to objects in other dictionaries more 
> instead of fully defining things.
Well this has been a good exercise in understanding what the MetaCRS 
project is about, certainly worth sticking around for.

Cheers,
Jody