A DA20 PLT INCORRECTLY SET THE LCL ALTIMETER BAROMETRIC PRESSURE AND MAY HAVE VIOLATED NEARBY CLASS C AIRSPACE.

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FLT DEPARTED ABOUT XA10 LCL TIME AND WAS A LCL PLEASURE FLT. INSTS WERE SET ACCORDING TO ATIS RPT. ALTIMETER WAS SET ACCORDING TO BAROMETRIC PRESSURE; BUT NO CHK WAS MADE WITH ACTUAL ARPT ALT. (MISTAKE #1) DURING CLBOUT; I SENSED THAT THE ALTIMETER DID NOT SEEM ACCURATE. OTHER INSTS INCLUDING AIRSPD AND RATE OF CLB LOOKED ACCURATE; SO I KNEW THE PITOT-STATIC SYS WAS WORKING PROPERLY. I DECIDED TO HEAD BACK TO THE CLASS D HOME ARPT. BECAUSE I WAS UP ONLY A SHORT WHILE 30 MINS; I DID NOT GET THE ATIS RPT FOR LNDG (MISTAKE #2). AFTER MY INITIAL RADIO CALL; HE DID NOT RESPOND FOR WHAT SEEMED TO BE A MIN. AS I WAS ABOUT TO RE-CONTACT THE CTLR; HE CAME BACK WITH A REQUEST TO IDENT. I DID SO; AND HE INSTRUCTED ME TO ENTER THE PATTERN FOR RWY 27L ON DOWNWIND AND RPT MID-FIELD. I DID NOT REQUEST AN ALT CHK; AS HE SEEMED BUSY (MISTAKE #3). WHILE MANEUVERING FOR DOWNWIND I DID NOT TRUST THE ALTIMETER; SO I USED JUDGEMENT OF THE GND TO ESTIMATE PATTERN ALT. THE DIFFICULTY HERE IS THAT MOST OF MY FLYING PRIOR TO RECENT EXPERIENCE WAS AT SMALLER; ONE RWY ARPTS. THE VISUAL CUES ARE MUCH DIFFERENT AND PATTERN ALT LOOKS RELATIVELY LOW BECAUSE OF THE LARGER SIZE OF THE ARPT. ZZZ1 INTL CLASS C AIRSPACE CUTS THROUGH ZZZ CLASS D AIRSPACE AT 2500 FT. ZZZ IS AT 905 FT MSL AND PATTERN ALT IS AT 1900 FT. I WAS SECURING THE ACFT; I NOTICED THAT I HAD SET THE WRONG BAROMETRIC PRESSURE (INSERTING THE TENTHS POS IN THE HUNDREDTHS POS) AND THAT THE RESULTANT ALTIMETER READING WAS APPROX 500 FT LOW. THE RESULTANT OF THIS STRING OF MISTAKES IS THAT I WAS ABOVE PATTERN ALT WHILE MANEUVERING TO LAND AND POSSIBLY MADE AN INCURSION INTO CLASS C AIRSPACE ABOVE THE ARPT.