FlightTaskAuto: reference update only if refence altitude has not bee set.

src/lib/FlightTasks/tasks/FlightTaskAuto.cpp

@@ -177,10 +177,13 @@ bool FlightTaskAuto::_isFinite(const position_setpoint_s sp)
 
 void FlightTaskAuto::_evaluateVehicleGlobalPosition()
 {
-	FlightTask::_evaluateVehicleLocalPosition();
-
 	// check if reference has changed and update.
-	if (_sub_vehicle_local_position->get().ref_timestamp != _time_stamp_reference) {
+	// Only update if reference timestamp has changed AND no valid reference altitude
+	// is available.
+	// TODO: this needs to be revisited and needs a more clear implementation
+	if (_sub_vehicle_local_position->get().ref_timestamp != _time_stamp_reference &&
+	    (_sub_vehicle_local_position->get().z_global && !PX4_ISFINITE(_reference_altitude))) {
+
 		map_projection_init(&_reference_position,
 				    _sub_vehicle_local_position->get().ref_lat,
 				    _sub_vehicle_local_position->get().ref_lon);

src/lib/FlightTasks/tasks/FlightTaskAuto.hpp

@@ -92,7 +92,7 @@ private:
 	map_projection_reference_s _reference; /**< Reference frame from global to local. */
 
 	map_projection_reference_s _reference_position{}; /**< Structure used to project lat/lon setpoint into local frame. */
-	float _reference_altitude = 0.0f;  /**< Altitude relative to ground. */
+	float _reference_altitude = NAN;  /**< Altitude relative to ground. */
 	hrt_abstime _time_stamp_reference = 0; /**< time stamp when last reference update occured. */
 
 	bool _evaluateTriplets(); /**< Checks and sets triplets. */