def write_gps_datum(self, code=None, gps_datum=None):
        """
        Write the mandatory GPS datum header::

            writer.write_gps_datum()
            # -> HFDTM100GPSDATUM:WGS-1984

            writer.write_gps_datum(33, 'Guam-1963')
            # -> HFDTM033GPSDATUM:Guam-1963

        Note that the default GPS datum is WGS-1984 and you should use that
        unless you have very good reasons against it.

        :param code: the GPS datum code as defined in the IGC file
            specification, section A8
        :param gps_datum: the GPS datum in written form
        """

        if code is None:
            code = 100

        if gps_datum is None:
            gps_datum = 'WGS-1984'

        self.write_fr_header(
            'DTM',
            '%03d' % code,
            subtype_long='GPSDATUM',
            value_long=gps_datum,
        )