AP_Declination: added get_earth_field_ga() interface

this gives a more convenient API for getting the earth field as a
vector

libraries/AP_Declination/AP_Declination.cpp

@@ -127,3 +127,19 @@ float AP_Declination::get_declination(float latitude_deg, float longitude_deg)
     return declination_deg;
 }
 
+/*
+  get earth field as a Vector3f in Gauss given a Location
+*/
+Vector3f AP_Declination::get_earth_field_ga(const Location &loc)
+{
+    float declination_deg=0, inclination_deg=0, intensity_gauss=0;
+    get_mag_field_ef(loc.lat*1.0e-7f, loc.lng*1.0e-7f, intensity_gauss, declination_deg, inclination_deg);
+
+    // create earth field
+    Vector3f mag_ef = Vector3f(intensity_gauss, 0.0, 0.0);
+    Matrix3f R;
+
+    R.from_euler(0.0f, -ToRad(inclination_deg), ToRad(declination_deg));
+    mag_ef = R * mag_ef;
+    return mag_ef;
+}

libraries/AP_Declination/AP_Declination.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <AP_Common/Location.h>
+
 /*
   magnetic data derived from WMM
  */
@@ -16,6 +18,11 @@ public:
     */    
     static bool get_mag_field_ef(float latitude_deg, float longitude_deg, float &intensity_gauss, float &declination_deg, float &inclination_deg);
 
+    /*
+      get earth field as a Vector3f in Gauss given a Location
+     */
+    static Vector3f get_earth_field_ga(const Location &loc);
+
     /*
       get declination in degrees for a given latitude_deg and longitude_deg
      */