 Alternatively, the corresponding map "T"2→"S"2 from the torus "T"2 to the sphere "S"2 (given by the point with given azimuth and elevation) is not a covering map at these points.Consider a case of a level-sensing platform on an aircraft flying due north with its three gimbal axes mutually perpendicular (i.e., roll, pitch and yaw angles each zero).If the aircraft pitches up 90 degrees, the aircraft and platform's yaw axis gimbal becomes parallel to the roll axis gimbal, and changes about yaw can no longer be compensated for.