SITL: add a hagl method to aircraft

libraries/SITL/SIM_Aircraft.cpp

@@ -132,12 +132,19 @@ float Aircraft::ground_height_difference() const
     return 0.0f;
 }
 
+/*
+   return current height above ground level (metres)
+*/
+float Aircraft::hagl() const
+{
+    return (-position.z) + home.alt*0.01f - ground_level - frame_height - ground_height_difference();
+}
 /*
    return true if we are on the ground
 */
 bool Aircraft::on_ground() const
 {
-    return (-position.z) + home.alt*0.01f <= ground_level + frame_height + ground_height_difference();
+    return hagl() <= 0;
 }
 
 /*

libraries/SITL/SIM_Aircraft.h

@@ -173,6 +173,9 @@ protected:
 
     bool on_ground() const;
 
+    // returns height above ground level in metres
+    float hagl() const; // metres
+
     /* update location from position */
     void update_position(void);