def npd_to_pmf(nodal_plane_dist, use_default=False):
    """
    Returns the nodal plane distribution as an instance of the PMF class
    """
    if isinstance(nodal_plane_dist, PMF):
        # Aready in PMF format - return
        return nodal_plane_dist
    else:
        if use_default:
            return PMF([(1.0, NodalPlane(0.0, 90.0, 0.0))])
        else:
            raise ValueError('Nodal Plane distribution not defined')