Compass: added COMPASS_AUTODEC option

when this is 1 (which is the default), we will get the declination
automatically via the AP_Declination library

when it is 0 we will use the value configured by the user

libraries/AP_Compass/Compass.cpp

@@ -7,6 +7,7 @@ const AP_Param::GroupInfo Compass::var_info[] PROGMEM = {
     AP_GROUPINFO("DEC",    2, Compass, _declination),
     AP_GROUPINFO("LEARN",  3, Compass, _learn), // true if learning calibration
     AP_GROUPINFO("USE",    4, Compass, _use_for_yaw), // true if used for DCM yaw
+    AP_GROUPINFO("AUTODEC",5, Compass, _auto_declination),
     AP_GROUPEND
 };
 
@@ -21,6 +22,7 @@ Compass::Compass(void) :
     _use_for_yaw(1),
     _null_enable(false),
     _null_init_done(false),
+    _auto_declination(1),
     _orientation(ROTATION_NONE)
 {
 }
@@ -57,23 +59,17 @@ Compass::get_offsets()
     return _offset;
 }
 
-bool
-Compass::set_initial_location(long latitude, long longitude, bool force)
+void
+Compass::set_initial_location(long latitude, long longitude)
 {
-	// If the user has choosen to use auto-declination regardless of the planner value
-	// OR
-	// If the declination failed to load from the EEPROM (ie. not set by user)
-	if(force || !_declination.load())
-	{
+    // if automatic declination is configured, then compute
+    // the declination based on the initial GPS fix
+	if (_auto_declination) {
 		// Set the declination based on the lat/lng from GPS
-		_declination.set(radians(AP_Declination::get_declination((float)latitude / 10000000, (float)longitude / 10000000)));
-
-		// Reset null offsets
 		null_offsets_disable();
+		_declination.set(radians(AP_Declination::get_declination((float)latitude / 10000000, (float)longitude / 10000000)));
 		null_offsets_enable();
-		return true;
 	}
-	return false;
 }
 
 void

libraries/AP_Compass/Compass.h

@@ -81,13 +81,12 @@ public:
 	///
 	virtual Vector3f &get_offsets();
 
-	/// Sets the initial location used to get declination - Returns true if declination set
+	/// Sets the initial location used to get declination
 	///
 	/// @param  latitude             GPS Latitude.
 	/// @param  longitude            GPS Longitude.
-	/// @param  force				 Force the compass declination update.
 	///
-	bool set_initial_location(long latitude, long longitude, bool force);
+	void set_initial_location(long latitude, long longitude);
 
 	/// Program new offset values.
 	///
@@ -129,6 +128,7 @@ protected:
 	AP_Float            _declination;
     AP_Int8             _learn;                 ///<enable calibration learning
     AP_Int8             _use_for_yaw;           ///<enable use for yaw calculation
+    AP_Int8             _auto_declination;      ///<enable automatic declination code
 
 	bool                _null_enable;        	///< enabled flag for offset nulling
 	bool                _null_init_done;        ///< first-time-around flag used by offset nulling