MOONEY 20 -PLT DSNDED BELOW INST APCH XING FIX DUE TO MISREADING HIS ACTUAL LOCATION ON GPS DEPICTION. HE NOTICED HIS ERROR AT THE TIME APCH CTLR WAS ALERTING HIM OF HIS ALT AND POS.

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I WAS FLYING INTO LEHIGH VALLEY INTL ARPT (ABE) UNDER IFR FLT PLAN. I HAD BEEN VECTORED ONTO FINAL APCH COURSE AND 'CLRED' FOR THE GPS-24 APCH. AT THE TIME CLRNC WAS GIVEN; I WAS ABOUT 10 MI FROM THE DSCNT POINT (WILEY) BUT HAD LOST SITUATIONAL AWARENESS; I WAS ON THE APCH COURSE AT 3000 FT; BUT DID NOT SEE THE DSCNT POINT (NAZAR) ON MY GPS. I WAS APCHING WILEY (SHOWN AS NEXT WAYPOINT) ON MY GPS AND ASSUMED I HAD PASSED NAZAR (A MISTAKE). SINCE I HAD BEEN CLRED FOR THE APCH; I STARTED MY DSCNT TO 1900 FT (THE ALT FOR WILEY). AS I WAS DSNDING (AT ABOUT 2100 FT) NAZAR CAME ONTO THE GPS SCREEN AND I REALIZED I WAS 900 FT LOW. AT THE SAME TIME; ABE APCH CALLED AND NOTED THAT I WAS AT NAZAR AND 2100 FT. THERE WAS NOTHING TO DO BUT BE EMBARRASSED; AND CONTINUE MY DSCNT TO 1900 FT. EVEN THOUGH IN MVFR; I HAD CLR SIGHT OF THE TERRAIN; SO AT NO TIME WAS THIS FLT IN JEOPARDY. BUT CLRLY IT COULD HAVE BEEN. ON REFLECTION; I SHOULD HAVE NOTICED/READ THE GPS DISTANCE FROM WILEY. MY APCH PLATE SHOWS NAZAR 5.1 MI FROM WILEY. SINCE I WAS ON COURSE; THE DME WOULD HAVE SHOWED ME MY POS RELATIVE TO NAZAR; AND I COULD HAVE PLANNED ACCORDINGLY. THIS PROB WILL NOT HAPPEN AGAIN.