def __get_ps_range(ps, field):
    """
    Get the minimum and maximum values of the field in a ParticleSet.

    Parameters
    ----------
    ps: ParticleSet
        A set of Lagrangian particles.
    field: str
        A column field in p.data, e.g., LON or LAT

    Returns
    -------
    re: tuple of float
        minimum and maximum values
    """
    data_min = []
    data_max = []

    for p in ps:
        data = p.records[field]

        data_min.append(data.min())
        data_max.append(data.max())
    
    dmin = min(data_min)
    dmax = max(data_max)
    
    return dmin, dmax