Plane: more accurate altitude tracking close to waypoints

don't use a glide slope if the altitude change is less than 15 meters

ArduPlane/altitude.pde

@@ -324,6 +324,15 @@ static void set_offset_altitude_location(const Location &loc)
         target_altitude.offset_cm = target_altitude.terrain_alt_cm - (height * 100);
     }
 #endif
+
+    // if we are within 15 meters of the target altitude then reset
+    // the offset to not use a glide slope. This allows for more
+    // accurate flight of missions where the aircraft may lose or gain
+    // a bit of altitude near waypoint turn points due to local
+    // terrain changes
+    if (labs(target_altitude.offset_cm) < 1500) {
+        target_altitude.offset_cm = 0;
+    }
 }
 
 /*