def get_swell_url(self, swell_type):
        """Get swell arrow url."""
        if swell_type not in SWELL_TYPES:
            raise ValueError('Invalid swell type: {}'.format(swell_type))
        key = "swell_components_{}_direction".format(swell_type)
        swell_direction = self.f_d.get(key)
        if swell_direction is not None:
            rounded = int(5 * round(float(swell_direction)/5))
            return SWELL_ARROW_URL.format(rounded)