I have QGIS with proj 4.8.0 installed. I'd like to add Winkel Tripel projection which seems to be defined as
But that definition string does not work. Is this a bug in my proj version, or what's the correct definition string?
Summing up the discussion above:
While Winkel Tripel projection is defined in the proj library and can be called from the command line, it can't be used as a custom CRS in QGIS because there's no inverse transformation in the proj library.
The enhancement request to add this functionality has been closed since it seems that the inverse transformation isn't trivial.