MLG TAKES OFF WITH A LARGE COMPASS ERROR CAUSING A 30 DEG COURSE DEV.

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ACFT WAS PARKED AT GATE X. WE DEPARTED ON TIME AT XA51 EDT; PUSHED OFF GATE; AND SINCE THERE WERE NO DELAYS; STARTED BOTH ENGS AND PROCEEDED TO RWY 31 FOR IMMEDIATE TKOF. INITIAL HDG WAS 340 DEGS. HDG BUGS WERE SET TO 340 DEGS AND SHORTLY AFTER TKOF; FO WHO WAS MAKING THE TKOF; STARTED A R TURN; BUT THEN CAME BACK L. I LOOKED AT MY COMPASS AND SAW THE HDG BUG WAS AT 340 DEGS AND IT WAS OVER ON THE L; NOT R. MY RMI ALSO SHOWED WITHIN 5 DEGS OF MY COMPASS; SO ASSUMED (INCORRECTLY) THAT WE WERE ON THE CORRECT HDG; AS BOTH OF OUR COMPASSES WERE WITHIN 5 DEGS OF EACH OTHER. JUST THEN NY DEP ASKED WHERE WE WERE GOING. I REPLIED THAT WE WERE ON A 340 DEG HDG; AS ASSIGNED. HE THEN STATED THAT NO; WE WERE WELL L OF THAT COURSE AND HDG FOR EWR AIRSPACE. WE STARTED AN IMMEDIATE R TURN AND FOLLOWED THE HUDSON RIVER UNTIL GIVEN A SUBSEQUENT L TURN TO THE SW. BY LOOKING DOWN AT THE RWY CONFIGN AT EWR I WAS ABLE TO DETERMINE THAT BOTH OUR COMPASSES WERE ABOUT 30 DEGS OFF. FO AND I MANUALLY SLEWED THE COMPASSES TO THE CORRECT HDG AND THEY BOTH OPERATED NORMALLY FOR THE REMAINDER OF THE FLT TO PHL. ITEM WAS WRITTEN UP IN ACFT LOG AND MAINT WAS UNABLE TO FIND ANY PROB WITH THE COMPASSES. FLT OPERATED NORMALLY TO ATL. WE WERE NOT SUSPECTING ANY PROBS AT FLT ORIGINATIONS; AS WE HAD BEEN FLYING THIS SAME ACFT ALL DAY WITH NO PROBS. THIS MORNING; AT ATL; I MENTIONED THIS TO THE CAPT WHO WAS GOING TO TAKE THIS SAME ACFT OUT; AND HE ADVISED ME THAT HE HAD HAD THE SAME PROB AT THIS GATE AT LGA ON ABOUT 3 PREVIOUS FLTS; WITH DIFFERENT ACFT. HE HAD MADE A SIMILAR RPT TO THE CHIEF PLT AT BOS. WE BELIEVE IT IS SOMETHING TO DO WITH THE GATE X AND GATE Y AREA OF THE LGA RAMP. SOMETHING IS CAUSING THE COMPASSES TO SLEW OFF COURSE BY 30 DEGS; AND IF NOT ENOUGH TAXI TIME IS ENCOUNTERED TO ALLOW THE COMPASSES TO RESET THEMSELVES; WE ALL NEED TO BE AWARE OF THIS AND TO MANUALLY CHK AND SLEW THE COMPASSES TO THE CORRECT HDG; IF NECESSARY; BEFORE TKOF. PERHAPS A WARNING PLACED ON THE JETWAY OR TERMINAL BUILDING; CLRLY VISIBLE TO THE FLC; AND/OR A NOTE ON THE 10-T PAGE OF THE MANUAL WOULD BE A HELP. APPARENTLY THIS HAS HAPPENED NUMEROUS TIMES BEFORE. I DON'T KNOW IF IT IS A PROB ON THE MLG; AND MLG ACFT ALONE; OR FLEET-WIDE; BUT I WOULD LIKE TO KNOW IF OTHER TYPES OF ACFT HAVE ENCOUNTERED THIS PROB.