def calculate_z_centr_NAP(df, z_id):
    """
    Assign z_centr with respect to NAP to each layer of a DataFrame.
    :param df: Original DataFrame.
    :param z_id: Elevation with respect to the NAP of my field.
    :return:  DataFrame with z_centr_NAP column.
    """
    df["z_centr_NAP"] = z_id - df["z_centr"]

    return df