EMJ EXPERIENCED MAGNETIC INSTABILITY ON DEP FROM LGA.

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WE HAVE BEEN INFORMED BEFORE BUT I THINK IT MAY BE NECESSARY TO ESTABLISH A PROC OR REITERATE THE INFO ABOUT THE AREA OF MAGNETIC INSTABILITY AT LGA APCHING THE DEP END OF RWY 13. AS WE TAXIED THROUGH THIS AREA; WE GOT HDG COMPARATORS FOLLOWED BY A MOMENTARY HDG FAIL MESSAGE. BOTH MESSAGES WENT AWAY WITHIN A FEW SECONDS AS WE TAXIED INTO POS. HAD THEY NOT; WE WOULD NOT HAVE ACCEPTED OUR DEP CLRNC. HOWEVER; SINCE ALL FLAGS HAD GONE AWAY; WE TOOK THE DEP ON THE ROLL AND TOOK OFF NORMALLY. WE WERE GIVEN THE CONEY CLB WHICH INSTRUCTS A 180 DEG HDG TO INTERCEPT THE CRI 044 DEG RADIAL (I BELIEVE). WE TURNED TO 180 DEGS AND WATCHED THE DME COUNT DOWN TOWARDS THE VOR BUT NEVER INTERCEPTED. ATC ASKED US TO CONFIRM WE WERE FLYING THE CONEY CLB WHICH WE AFFIRMED AND WERE HANDED OFF TO THE NEXT CTLR. WE PASSED CRI AT APPROX 2 DME AND NEVER INTERCEPTED THE RADIAL; AND WERE SHOWING 160 KTS OF WIND BTWN 3000-5000 FT. THIS DIDN'T SEEM RIGHT SO I XCHKED THE MAGNETIC COMPASS AND IT SHOWED BOTH HDG INDICATIONS WERE APPROX 20 DEGS OFF. WE WERE NEARLY PARALLELING OUR COURSE. WE TOLD ATC WE WERE EXPERIENCING HDG IRREGULARITIES AND WERE WORKING ON CORRECTING THEM. WE BOTH RESET OUR HDGS AND SLAVED THEM AND THEY CAME BACK IN AGREEMENT WITH THE MAGNETIC COMPASS AND ATC CONFIRMED WE WERE ON THE ASSIGNED HDG. ATC DIDN'T SEEM TO HAVE A PROB WITH US; BUT IT SEEMS THAT THE AREA THAT THREW OFF OUR HDG INDICATION COULD PRESENT A SERIOUS PROB SINCE NO FLAG OR OTHER NOTICEABLE INDICATION WAS GIVEN WHEN BOTH HDG INDICATORS WERE OFF BUT WITHIN THE 5 DEG DIFFERENCE TOLERANCE.