ACFT EQUIP PROB CAUSED BY ARPT PROB RAMP STANDARDS.

Narrative

OUR CLRNC OUT OF LGA REQUIRED A TURN TO A HDG OF 360 DEGS AFTER TKOF ON RWY 31. OUR GATE IS VERY CLOSE TO THE DEP END OF RWY 31. START UP; CHKLISTS; AND TAXI INVOLVED LESS THAN 4 MINS AND WE WERE CLRED FOR TKOF UPON REACHING THE END OF THE DEP RWY. DURING THE TKOF ROLL I NOTED THAT MY HSI READ 350 DEG WHEN IT SHOULD BE READING 310 DEGS (RWY HDG). THE CAPT'S HSI AND BOTH OUR RMI'S READ THE SAME ERRONEOUS HDG. NO FLAGS OR INST FAILURE WARNINGS WERE PRESENT. WITH SOME HELP FROM DEP CTL; WE MANAGED TO GET ON OUR CORRECT HDG AND SUBSEQUENTLY RE-SYNCED THE HSI'S AGAINST THE WET COMPASS. ALL FURTHER OPS WERE NORMAL BUT WE MAY HAVE SET OFF SOME NOISE MONITORS DUE TO OUR INITIAL CONFUSION ABOUT OUR HDG. WE LEARNED LATER THAT THE GATE WE HAD PARKED AT PRIOR TO OUR DEP FROM LGA HAD PRODUCED GROSS COMPASS SWINGS IN THE PAST ON SOME ACFT. EVIDENTLY SOME MAGNETIC ANOMALY IS PRESENT THERE; PRODUCING AS MUCH AS 40 DEGS OF COMPASS SWING. A SUBSEQUENT RAPID DEP DOES NOT GIVE THE COMPASS SYS TIME TO RE-SYNC TO CORRECT HDG AND IF THE CREW DOESN'T CATCH IT (WE DIDN'T) A PROB AFTER DEP CAN DEVELOP. OUR COMPANY HAS SINCE ISSUED A NOTAM IN OUR RELEASE PAPERS THAT WARNS AGAINST COMPASS SWING POSSIBILITY AT THAT PARTICULAR GATE.