HDG TRACK DEV ON DEP WHICH WAS CAUGHT BY ALERT DEP CTLR VIGILANCE.

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AFTER TKOF WE ENGAGED THE RNAV TO THE AUTOPLT. ALMOST IMMEDIATELY WE GOT THE 'WHERE ARE YOU GOING?' FROM DEP CTL. ACCORDING TO OUR MAP DISPLAY; WE WERE PROCEEDING CORRECTLY; BUT DEP GAVE US VECTORS IN ALMOST THE OPPOSITE DIRECTION. IT TOOK QUITE AWHILE TO DECIPHER THE PROBLEM; WHAT WITH WORKLOAD; LOSS OF POSITIONAL AWARENESS; ETC; BUT IT APPEARED THAT I MISENTERED THE COORDS ON THE RAMP AND HAD A 2 DEG LONGITUDE ERROR. I THOUGHT THE CAPT HAD XCHKED IT BUT APPARENTLY HE NEVER DID. WE HAD OTHER IRS PROBLEMS DURING ALIGNMENT; SO WE DIDN'T CATCH THE POS ERROR AND THE IRS ACCEPTED IT. IT DIDN'T AT FIRST; BUT DUE TO THE OTHER PROBLEMS; WE MISSED THE POS PROBLEM. APPARENTLY; THE LGT IRS SYS WILL ACCEPT A GROSS ERROR AFTER A FEW TRIES. IN ANY EVENT; WE NAVIGATED THE REST OF THE TRIP THE OLD-FASHIONED WAY -- THANKFULLY; WE WERE NOT HEADED OVER THE OCEAN. ALSO; FORTUNATELY; ALTHOUGH WE STRAYED OFF COURSE; AND HAD A TFC CONFLICT; THERE WAS NO LOSS OF SEPARATION; THINKS TO A SHARP CTLR. ALL OF THIS COULD HAVE BEEN PREVENTED IF: I HAD BEEN MORE CAREFUL; THE CAPT HAD XCHKED; OR WE HAD BACKED UP THE DEP WITH VOR; ALL OF WHICH WE NORMALLY DO! GUESS IT ALL BOILS DOWN TO COMPLACENCY. ALSO; IT WOULD HELP IF THE IRS SYS WOULD TELL YOU IF YOUR POS WAS WAY OFF FROM THE LAST POS; SO THAT THE ERROR WOULD NOT BE MASKED BY OTHER ALIGNMENT PROBLEMS. LAST BUT NOT LEAST; AN UPDATE FEATURE WOULD BE VERY HELPFUL. IF WE HAD BEEN LAUNCHING ON AN OCEANIC XING; WE WOULD HAVE HAD TO LAND AND REALIGN.