add a point via latitude/longitude/altitude

def add_latlonalt(self, lat, lon, altitude, terrain_alt=False):
        '''add a point via latitude/longitude/altitude'''
        if terrain_alt:
            frame = mavutil.mavlink.MAV_FRAME_GLOBAL_TERRAIN_ALT
        else:
            frame = mavutil.mavlink.MAV_FRAME_GLOBAL_RELATIVE_ALT
        p = mavutil.mavlink.MAVLink_mission_item_message(self.target_system,
                                                         self.target_component,
                                                         0,
                                                         frame,
                                                         mavutil.mavlink.MAV_CMD_NAV_WAYPOINT,
                                                         0, 0, 0, 0, 0, 0,
                                                         lat, lon, altitude)
        self.add(p)