Copter: use terrain alt for precland if rangefinder is unavailable

ArduCopter/precision_landing.cpp

@@ -15,13 +15,15 @@ void Copter::init_precland()
 
 void Copter::update_precland()
 {
-    float final_alt = current_loc.alt;
+    int32_t height_above_ground_cm = current_loc.alt;
 
-    // use range finder altitude if it is valid
+    // use range finder altitude if it is valid, else try to get terrain alt
     if (rangefinder_alt_ok()) {
-        final_alt = rangefinder_state.alt_cm;
+        height_above_ground_cm = rangefinder_state.alt_cm;
+    } else if (terrain_use()) {
+        current_loc.get_alt_cm(Location_Class::ALT_FRAME_ABOVE_TERRAIN, height_above_ground_cm);
     }
 
-    copter.precland.update(final_alt);
+    copter.precland.update(height_above_ground_cm);
 }
 #endif