Narrative:

Departed las in map mode using LNAV. FMC programmed for oasis departure. FMC showed us crossing oasis and directed a right turn. ATC advised that we began our turn 4 mi short of oasis. We crosschecked with VOR and confirmed that we, in fact, were approximately 4 mi north of course. A few mins later the map corrected and was correct for remainder of flight. Callback conversation with reporter revealed the following information: reporter says he may have used the wrong term when he said 'map shift.' he said that the FMC turned them toward their next fix 4 mi before oasis intersection. He cannot swear that a FMC insertion error was not made but he doesn't think so. He did give an example: cleared to taxi into position on runway 25R at lax and the map would show them in position on runway 25L.

Related reports:

Google
 

Original NASA ASRS Text

Title: MAP SHIFT CAUSED AN EARLY TURN ON THE SID OUT OF LAS.

Narrative: DEPARTED LAS IN MAP MODE USING LNAV. FMC PROGRAMMED FOR OASIS DEP. FMC SHOWED US XING OASIS AND DIRECTED A R TURN. ATC ADVISED THAT WE BEGAN OUR TURN 4 MI SHORT OF OASIS. WE XCHKED WITH VOR AND CONFIRMED THAT WE, IN FACT, WERE APPROX 4 MI N OF COURSE. A FEW MINS LATER THE MAP CORRECTED AND WAS CORRECT FOR REMAINDER OF FLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAYS HE MAY HAVE USED THE WRONG TERM WHEN HE SAID 'MAP SHIFT.' HE SAID THAT THE FMC TURNED THEM TOWARD THEIR NEXT FIX 4 MI BEFORE OASIS INTXN. HE CANNOT SWEAR THAT A FMC INSERTION ERROR WAS NOT MADE BUT HE DOESN'T THINK SO. HE DID GIVE AN EXAMPLE: CLRED TO TAXI INTO POS ON RWY 25R AT LAX AND THE MAP WOULD SHOW THEM IN POS ON RWY 25L.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.