Plane: fixed an issue with landing on rising ground

when landing on rising ground we don't want to use the terrain look
fwd correction for the flare calculation as otherwise we will flare
too early (and thus too high)

ArduPlane/Plane.h

@@ -559,6 +559,9 @@ private:
 
         // are we doing loiter mode as a VTOL?
         bool vtol_loiter;
+
+        // how much correction have we added for terrain data
+        float terrain_correction;
     } auto_state;
 
     struct {

ArduPlane/altitude.cpp

@@ -595,6 +595,7 @@ void Plane::rangefinder_terrain_correction(float &height)
     }
     float correction = (terrain_amsl1 - terrain_amsl2);
     height += correction;
+    auto_state.terrain_correction = correction;
 #endif
 }
 

ArduPlane/commands_logic.cpp

@@ -230,7 +230,11 @@ bool Plane::verify_command(const AP_Mission::Mission_Command& cmd)        // Ret
 
         } else {
             // use rangefinder to correct if possible
-            const float height = height_above_target() - rangefinder_correction();
+            float height = height_above_target() - rangefinder_correction();
+            // for flare calculations we don't want to use the terrain
+            // correction as otherwise we will flare early on rising
+            // ground
+            height -= auto_state.terrain_correction;
             return landing.verify_land(prev_WP_loc, next_WP_loc, current_loc,
                 height, auto_state.sink_rate, auto_state.wp_proportion, auto_state.last_flying_ms, arming.is_armed(), is_flying(), rangefinder_state.in_range);
         }