No projection definition for code OGC:CRS84

[ckan]

No projection definition for code OGC:CRS84

[torfsen]

OK, so I think I have figured this out. It's complicated, so for future reference I'll document what I have found.

OGC:CRS84 (or, more precisely, urn:ogc:def:crs:OGC:1.3:CRS84) is the identifier of a coordinate reference system (CRS). According to the current GeoJSON standard (RFC 7946), OGC:CRS84 is the CRS used by GeoJSON. However, earlier versions of the GeoJSON standard (in particular the widely used 2008 standard) allow GeoJSON files to specify a custom CRS. That's what my GeoJSON file which caused the problem does:

 "crs": {
        "properties": {
            "name": "urn:ogc:def:crs:OGC:1.3:CRS84"
        },
        "type": "name"
},

So basically, my GeoJSON file simply explicitly states "Just use the default CRS". However, proj4leaflet sees the custom CRS declaration and passes it own to proj4js which doesn't have that CRS built-in. That's where the exception happens.

Fortunately, the solution is rather easy: proj4js does come with the EPSG:4326 CRS built-in, which differs from OGC:CRS84 only in the order of the coordinates (lon, lat vs. lat, lon). Since proj4js assumes lon, lat order anyways, we can just add OGC:CRS84 as an alias for EPSG:4326:

proj4.defs['OGC:CRS84'] = proj4.defs['EPSG:4326'];

This fixes the problem for me. Since the proj4leaflet team doesn't want to include that alias in proj4leaflet I think it would be nice to include in ckanext-geoview instead. I'll prepare a PR for that.