def earth_model():
    """
    Define the constants from the WGS-84 ellipsoidal Earth model.


    Parameters
    ----------
    None

    Returns
    -------
    a : semi-major axis of the Earth ellipsoid model
    f : flattening

    Notes
    -----
    The World Geodetic System (WGS) is a standard for use in cartography, geodesy, and navigation.
    The latest revision is WGS-84.

    """
    a = 6378137.0
    f = 1.0 / 298.257223563
    return a, f
