I believe I understand this to be occuring in one area at one airport. If so, do the affected aircraft have a remote compass system using flux valves (remote compass transmitters) in the wing tips? Would the compass error reverse if the other side of the aircraft was flown past the anomalie? I do know that B 707's, 727's and L-1011's will show an unpredictable and often LARGE compass spread if vehicles, etc. are under or passing by wingtips. I suppose a large iron transmitting tower could affect aircraft passing by. Also, how long, or to what distance from the anomalie does the compass error remain?