Convert x,y,z representation of points *on the unit sphere* of the
    spherical triangulation to lon / lat (radians).

    Note - no check is made here that (x,y,z) are unit vectors

def xyz2lonlat(x,y,z):
    """
    Convert x,y,z representation of points *on the unit sphere* of the
    spherical triangulation to lon / lat (radians).

    Note - no check is made here that (x,y,z) are unit vectors
    """

    xs = np.array(x)
    ys = np.array(y)
    zs = np.array(z)

    lons = np.arctan2(ys, xs)
    lats = np.arcsin(zs)

    return lons, lats