PLT OF PA28 SETS ALTIMETER KOLLSMAN INCORRECTLY AND FLIES INITIAL STAGES OF FLT 800 FT LOWER THAN INDICATED.

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THE ALTIMETER SETTING HAD CHANGED APPROX 1 INCH SINCE THE LAST TIME I FLEW THE AIRPLANE. THE ARPT THAT I FLY FROM IS UNCTLED AND NO CURRENT ALTIMETER SETTING IS AVAILABLE. I DID MY PREFLT AND TAXIED TO THE END OF THE TKOF RWY; DID A RUN-UP; CHKED ENG AND FLT INSTS; AND SET ALTIMETER TO FIELD ELEVATION 780 FT (I THOUGHT). I SHOULD SAY; SET THE LARGE POINTER TO 80 FT WITHOUT REGARD FOR THE SMALL POINTER; WHICH WAS AT '1;' WHICH MADE MY ALTIMETER READ 1780 FT. I TOOK OFF AND THOUGHT 'HOW DID I GET TO 2500 FT SO FAST?' AS I WAS TRYING TO FIGURE OUT WHAT WAS GOING ON; I HAD TO DSND TO BELOW 2100 FT TO GO BELOW CLASS C AIRSPACE. GND ELEVATION AT THIS POINT WAS ABOUT 500-600 FT MSL. SO I WAS FLYING AT ABOUT 1100 FT MSL; NOT EXACTLY SURE. AT THIS POINT I COULD SEE THAT I WAS A LOT LOWER THAN I THOUGHT; TUNED IN ATIS FROM LARGE ARPT AND GOT CORRECT ALTIMETER SETTING. IT WAS SET ABOUT .8 OF AN INCH OFF. I BELIEVE THAT I WAS OVER SPARSELY POPULATED AREA SO; THEREFORE; I DON'T THINK I VIOLATED ANY FARS. HOWEVER; I FILED THIS RPT BECAUSE IT WAS SOMETHING THAT COULD HAVE PRESENTED A VERY DANGEROUS SIT AND MAYBE SOMEONE ELSE CAN LEARN TO ALWAYS CHK THE ALTIMETER COMPLETELY WHEN SETTING IT. I KNOW I WILL.