def _get_zone_letter(latitude):
    """
    Calculate the UTM zone letter from the given latitude. Shamelessly lifted
    from the utm.latitude_to_zone_letter function.

    Parameters
    ----------
    lat : float
        Latitude

    Returns
    -------
    zone_letter : str
        Zone letter

    """

    ZONE_LETTERS = "CDEFGHJKLMNPQRSTUVWXX"

    if -80 <= latitude <= 84:
        return ZONE_LETTERS[int(latitude + 80) >> 3]
    else:
        return None