CORP MLG FAILS TO THOROUGHLY CHK GND POS ON INS. MUST RETURN LAND.

Narrative

DEPARTING PANC FOR RJ55. PRIOR TO DEP; LOADED RAMP COORDINATES INTO INS. DOUBLE-CHKED DATA AND THEN X FILED INTO #2 INS PER COMPANY SOP. THIS WAS A QUICK TURN/CREW CHANGES-INBOUND CREW HAD NO NEGATIVE COMMENTS ABOUT ACFT. I DID NOT CHK FOR INBOUND ERRORS IN INS POS. RECEIVED ATC CLRNC. BOTH PLTS LOADED FLT PLAN INTO FMS AND XCHKED TO ENSURE THE INS POS UPDATE HAD 'TAKEN' (DONG SO WAS NOT PART OF OUR SOP -- JUST THE FLT PLAN). AFTER TKOF AND LEAVING 11000 FT; ATC CLRED US DIRECT TO A FIX. I SELECTED THE FIX ON MY INS/FMS -- SOMETHING DIDN'T LOOK RIGHT. INVESTIGATION SHOWED THAT BOTH INS'S WERE OFF BY 57 NM (.95 LONG PLUS A WEE BIT OFF LATERALLY). CAN'T UPDATE INS IN FLT IN THIS INSTALLATION -- NO CHOICE BUT RETURN TO PANC AND UPDATE ON GND. MADE OVERWT LNDG. MORAL: WE ARE CHANGING SOP TO HAVE BOTH PLTS VERIFY RAMP POS -- XCHKING RAMP THE SAME AS THE FLT PLAN. OTHER FACTORS (NO EXCUSES) INBOUND CREW HAD HAD TROUBLE WITH THE FMS COMPUTED POS; DUE TO VLF INPUT; AND DIDN'T INFORM US (OUTBOUND CREW).