Get the central longitude from any transformation / projection

[asinghvi17]

It's possible to do this arbitrarily by using Proj:

PJ *proj_get_prime_meridian(PJ_CONTEXT *ctx, const PJ *obj)
Get the prime meridian of a CRS or a GeodeticReferenceFrame.

The returned object must be unreferenced with proj_destroy() after use. It should be used by at most one thread at a time.

Parameters
:
ctx -- PROJ context, or NULL for default context

obj -- Object of type CRS or GeodeticReferenceFrame (must not be NULL)

Returns
:
Object that must be unreferenced with proj_destroy(), or NULL in case of error.

int proj_prime_meridian_get_parameters(PJ_CONTEXT *ctx, const PJ *prime_meridian, double *out_longitude, double *out_unit_conv_factor, const char **out_unit_name)
Return prime meridian parameters.

Parameters
:
ctx -- PROJ context, or NULL for default context

prime_meridian -- Object of type PrimeMeridian (must not be NULL)

out_longitude -- Pointer to a value to store the longitude of the prime meridian, in its native unit. or NULL

out_unit_conv_factor -- Pointer to a value to store the conversion factor of the prime meridian longitude unit to radian. or NULL

out_unit_name -- Pointer to a string value to store the unit name. or NULL

Returns
:
TRUE in case of success.

[asinghvi17]

Unfortunately this does not seem to return the lon_0 but rather the PRIMEM as defined in WKT, so it's a non starter. Maybe we need to emit the Proj string of the target CRS, and parse that...but then we'd also need to check the source CRS!