def _surf70(phi1, phi2, phi3, phi4):
    """Compute area of a south fire trapeze

    Parameters
    ----------
    phi1 : float
        Level-set at south west point
    phi2 : float
        Level-set at south east point
    phi3 : float
        Level-set at north east point
    phi4 : float
        Level-set at north west point

    Returns
    -------
    float
        SGBA
    """
    return 0.5 * ((0.5 - phi1) / (phi4 - phi1) + (0.5 - phi2) / (phi3 - phi2))