Plane: Added a method that returns current altitude relative to terrain, if available, or home otherwise.

ArduPlane/Plane.h

@@ -810,6 +810,7 @@ private:
     int32_t get_RTL_altitude();
     float relative_altitude(void);
     int32_t relative_altitude_abs_cm(void);
+    float relative_ground_altitude(void);
     void set_target_altitude_current(void);
     void set_target_altitude_current_adjusted(void);
     void set_target_altitude_location(const Location &loc);

ArduPlane/altitude.cpp

@@ -132,6 +132,20 @@ int32_t Plane::relative_altitude_abs_cm(void)
     return labs(current_loc.alt - home.alt);
 }
 
+/*
+  return relative altitude in meters (relative to terrain, if available,
+  or home otherwise)
+ */
+float Plane::relative_ground_altitude(void)
+{
+#if AP_TERRAIN_AVAILABLE
+    float altitude;
+    if (terrain.status() == AP_Terrain::TerrainStatusOK && terrain.height_above_terrain(altitude, true)) {
+        return altitude;
+    }
+#endif
+    return relative_altitude();
+}
 
 /*
   set the target altitude to the current altitude. This is used when