Get the EPSG code or URN of an existing CRS

The identifier of a Coordinate Reference System (CRS) object can be obtained by the getIdentifiers() method, which usually return a collection of zero or one element. If the CRS has been created from a Well Known Text (WKT) parsing and the WKT ends with an AUTHORITY["EPSG", "xxxx"] (WKT version 1) or ID["EPSG", xxxx] (WKT version 2) element, then the identifier (an EPSG numerical code in this example) is the xxxx value in that element. If the CRS has been created from the EPSG geodetic dataset (for example by a call to CRS.forCode("EPSG:xxxx")), then the identifier is the xxxx code given to that method. If the CRS has been created in another way, then the collection returned by the getIdentifiers() method may or may not be empty depending if the program that created the CRS took the responsibility of providing identifiers.

If the collection of identifiers is empty, the most effective fix is to make sure that the WKT contains an AUTHORITY or ID element (assuming that the CRS was parsed from a WKT). If this is not possible, then the org.apache.sis.referencing.IdentifiedObjects class contains some convenience methods which may help. In the following example, the call to lookupEPSG(…) will scan the EPSG database for a CRS equals (ignoring metadata) to the given one. Note that this scan is sensitive to axis order. Most geographic CRS in the EPSG database are declared with (latitude, longitude) axis order. Consequently if the given CRS has (longitude, latitude) axis order, then the scan is likely to find no match.

Direct dependencies

Maven coordinates	Module info	Remarks
`org.apache.sis.storage:sis-referencing`	`org.apache.sis.referencing`
`org.apache.sis.non-free:sis-embedded-data`	`org.apache.sis.referencing.database`	Optional. Non-Apache license.

The EPSG dependency is not needed if the WKT string declares an AUTHORITY element. But it is required if the AUTHORITY element is absent and Apache SIS needs to scan the EPSG database for finding its value.

Code example

import org.opengis.referencing.crs.CoordinateReferenceSystem;
import org.opengis.util.FactoryException;
import org.apache.sis.referencing.CRS;
import org.apache.sis.referencing.IdentifiedObjects;

public class LookupAuthorityCode {
    /**
     * Demo entry point.
     *
     * @param  args  ignored.
     * @throws FactoryException if an error occurred while creating the CRS or searching in EPSG database.
     */
    public static void main(String[] args) throws FactoryException {
        CoordinateReferenceSystem crs = CRS.fromWKT(
                """
                PROJCRS["NTF (Paris) / zone to be discovered by the demo",
                  BASEGEODCRS["NTF (Paris)",
                    DATUM["Nouvelle Triangulation Francaise",
                      ELLIPSOID["Clarke 1880 (IGN)", 6378249.2, 293.4660212936269]],
                      PRIMEM["Paris", 2.5969213],
                    UNIT["grade", 0.015707963267948967]],
                  CONVERSION["Lambert zone II",
                    METHOD["Lambert Conic Conformal (1SP)"],
                    PARAMETER["Latitude of natural origin", 52.0],
                    PARAMETER["Longitude of natural origin", 0.0],
                    PARAMETER["Scale factor at natural origin", 0.99987742],
                    PARAMETER["False easting", 600000.0],
                    PARAMETER["False northing", 2200000.0]],
                  CS[Cartesian, 2],
                    AXIS["Easting (E)", east],
                    AXIS["Northing (N)", north],
                    LENGTHUNIT["metre", 1],
                  REMARK["EPSG:27572 identifier intentionally omitted."]]
                """);

        System.out.println("Identifier declared in the CRS: "
                + IdentifiedObjects.getIdentifier(crs, null));

        System.out.println("Searching in EPSG database: "
                + IdentifiedObjects.lookupEPSG(crs));

        System.out.println("Same, but more generic: "
                + IdentifiedObjects.lookupURN(crs, null));
    }
}

Output

Identifier declared in the CRS: null
Searching in EPSG database: 27572
Same, but more generic: urn:ogc:def:crs:EPSG:9.9.1:27572