2 PA28 PLTS ON INST PRACTICE FLT BECOME DISORIENTED OVER LIN WHEN RELYING ON GPS.

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CLRNC WAS: DIRECT LINDEN AND DEPART LINDEN 340 DEGS AT 6000 FT. WE MISSED VOR BY 7 MI AND 200 FT ALT. WHY? GENERALLY: STUPIDITY; IN THAT I WAS LULLED INTO FOLLOWING GPS INDICATIONS INSTEAD OF VOR INDICATIONS. AFTER DEPARTING MODESTO ARPT ON RADAR VECTORS; TOLD BY ATC TO GO DIRECT LIN WHEN ABLE. STARTED OUT OK. THEN PUT LIN INTO GARMIN 195 GPS. THE PROB WAS: LIN; WHICH TURNED OUT TO BE APPROX 7 MI SW OF LIN VOR. INSTEAD OF FOLLOWING VOR READINGS; WATCHED GPS (STUPID). WHEN WE GOT TO LIN; SWITCHED HOODS AND SAFETY PLT TURNED AIRPLANE TO 340 DEGS WHILE I ADJUSTED HOOD. ALMOST IMMEDIATELY ATC CALLED FOR OUR POS. ALT WENT TO 6200 FT (WHICH THEY THEN ALSO REQUESTED OUR ALT). CORRECTED IMMEDIATELY AND DETERMINED WE WERE NOT AT LIN. MADE APPROPRIATE CORRECTIONS. SURE ATC WAS SIGHING AND SHAKING HEADS TO ALL THIS. PROB: NEXT TIME; USE VOR INDICATORS AS PRIMARY AND GPS AS SUPPLEMENTAL INFO. ALSO; LIN DID NOT HAVE VOR SYMBOL ON THE SCREEN; WHICH SHOULD ALSO HAVE ALERTED ME THAT WE DID NOT HAVE THE VOR WAYPOINT. SUPPLEMENTAL INFO FROM ACN 459211: HAD THIS BEEN AN IFR FLT WITH A GPS AS OUR PRIMARY MEANS OF NAV IN A NON RADAR ENVIRONMENT AND WITH DIFFERENT TERRAIN; WE MIGHT NOT BE HERE TO FILE THIS RPT. A GPS IS A COMPLEX PIECE OF EQUIP; AND EVEN WITH A DECENT HUMAN FACTORS DESIGN ON THE KEYBOARD AND DISPLAY; THERE ARE MANY WAYS TO MAKE MISTAKES. WE'VE UNINTENTIONALLY FOUND A FEW. IT IS OBVIOUS THAT WE MUST CONDUCT RECURRENT TRAINING ON THE GPS TO KEEP THE COMPLEXITY OF THIS DEVICE TAMED. SINCE THERE ARE MULTIPLE FIXES THAT BEGIN WITH LIN IN THE DATABASE; ONE MUST BE CAREFUL TO CONFIRM EACH KEYSTROKE IN SELECTING THE FIX. WE SPENT SOME TIME ON THE GND AFTER THIS INCIDENT TO DETERMINE THE CAUSE OF THE POSITIONAL ERROR ON THE GPS.