def calculate_zf_NAP(df, z_id):
    """
    Assign the zf with respect to NAP to each layer of a DataFrame.

    :param df: Original DataFrame.
    :param z_id: (float) Elevation with respect to the NAP of my field.
    :return: DataFrame with zf_NAP column.
    """
    df["zf_NAP"] = z_id - df["zf"]

    return df