AN SMT PISTON FREIGHTER HAD A PROBLEM WITH ITS PITOT STATIC SYS. IT CHOSE TO CONTINUE ITS TRIP IN VMC.

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I HAD TAKEN OFF FROM JANESVILLE; WI; CLBING OUT OF 1200 FT; WHEN I REALIZED THAT THE AIRSPD INDICATOR WAS INDICATING INCORRECTLY. IT WAS VFR; AND TOPS WERE RPTED TO BE 5000 FT IN THAT AREA. I CONTINUED CLBING AND RECYCLED THE PITOT HEAT. AT APPROX 4000 FT; CHICAGO CTR REQUESTED MY ALT. I TOLD ATC TO STANDBY. THEY RESPONDED THAT THEY WOULD LIKE ME TO RPT MY ALT EVERY 1000 FT; AND TO EXPEDITE TO 11000 FT DUE TO MILWAUKEE AIRSPACE. AT THAT TIME I RESPONDED TO ATC THAT I WAS HAVING AN ALTIMETER PROBLEM AND COULD NOT MAKE THAT RPT. ATC ASKED ME IF I COULD ESTIMATE MY ALT. THE TOPS HAD JUST BEEN RPTED AT 9000 FT; AND SINCE I WAS WELL ON TOP; I ESTIMATED 10500 FT. ATC ASKED IF I WOULD LIKE TO CONTINUE; AND I SAID YES. A FEW MINS LATER A CTLR TOLD ME THEY COULD NOT GUARANTEE MY ALT. SHE THEN TOLD ME I COULD CONTINUE VFR ON TOP IF I COULD MAINTAIN VFR; AND IF THIS WERE NOT POSSIBLE; TO LET THEM KNOW. AFTER LEVELING OFF; I CYCLED THE ALTERNATE STATIC AIR SOURCE; AND THE ALTIMETER READING INCREASED 3000 FT. IT WAS NOW INDICATING 13000 FT. ATC ASKED ME TO VERIFY MY ALT; AND I RPTED 13000 FT. ATC THEN ASKED IF I COULD DSND TO 12500 FT; AND I RESPONDED THAT I COULD. I BEGAN A TIMED 500 FPM DSND TO 12500 FT. I CONTINUED THE FLT AS WE CROSSED LAKE MICHIGAN; AND ON NEARING GRAND RAPIDS; MI; AN ACR COMMUTER CALLED ATC AND ASKED IF THEY WERE SHOWING ANY TFC IN HIS 12 O'CLOCK POS AND 1700 FT ABOVE HIM. ATC RESPONDED IN THE NEGATIVE; BUT SAID THEY DID HAVE A TWIN SMT AT 10500 FT; AND ASKED THE COMMUTER TO STAND BY. AGAIN ATC RESPONDED THAT THEY HAD A TWIN SMT WITH AN ESTIMATED ALT OF 10500 FT. THE COMMUTER SAID THE TFC HAD JUST COME UP ON HIS TCASII AND WAS INDICATING 12500 FT. I LOOKED OFF MY R WING AND SAW THE COMMUTER; AND TOLD ATC I COULD CONFIRM HE WAS BELOW US AND THAT THE ALT APPEARED TO CHK.