[geotk] Finding a CRS code/URN from a CoordinateReferenceSystem object

[Martin Desruisseaux]

Hello Calum

Calum.Byrom at tessella.com a écrit :
> Thanks for the quick response.  As requested, crs.toString() gives:
> 
> GEOGCS["ED50",
>   DATUM["European Datum 1950",
>     SPHEROID["International 1924", 6378388.0, 297.0]],
>   PRIMEM["Greenwich", 0.0],
>   UNIT["degree", 0.017453292519943295],
>   AXIS["Longitude", EAST],
>   AXIS["Latitude", NORTH]]
> 
> (...snip...)
> 
> the test data returns 'null' for CRS.lookupIdentifier(Citations.EPSG, 
> crs, true) - and for any combinations of the input parameters;

In the EPSG database, the axis order for geographic CRS is (latitude, 
longitude). The CRS posted above has the opposite axis order (as we can see from 
the AXIS[...] elements). Consequently by default the lookupIdentifier methods 
consider that the given CRS doesn't match the definition in the EPSG database.

It is possible to force the library to treat axis order as if it was (longitude, 
latitude) despite what EPSG define. There is two way of doing that: you can set 
the (longitude, latitude) axis order on a system-wide basis (convenient if you 
are sure that all your data are that way, but not recommanded if you need to 
deal with latest OGC standards like WMS 1.3.0 and above):

   Hints.putSystemDefault(Hints.FORCE_LONGITUDE_FIRST_AXIS_ORDER, Boolean.TRUE);

or you can force the (longitude, latitude) axis order on a case-by-case basis:

   String code = ((AbstractAuthorityFactory) 
CRS.getAuthorityFactory(true)).getIdentifiedObjectFinder(CoordinateReferenceSystem.class).findIdentifier(crs);


> the 'real' data returns 'CRS:84' (what I'm actually looking for!) for 
> CRS.lookupIdentifier(crs, true) but 'null' for any other combination of 
> inputs.

CRS:84 is defined by the WMS specification as (longitude, latitude) - at the 
opposite of what EPSG define - so this matches your CRS. Other combinaison of 
parameters will consider that there is no match if the specified authority is 
Citations.EPSG, unless axis order has been forced as described above.


	Martin

P.S.: I may be away of internet for the next week, since I'm traveling to the 
OGC meeting. So I may be slow in answering the next emails.