SMA PLT TOOK OFF ON WRONG RWY.

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ON APCH TO LAND AT OPF; THE ATIS GAVE RWY 9L AS THE RWY IN USE; BUT I WAS OFFERED RWY 12 AS THE WIND WAS MORE FAVORABLE FOR THAT RWY. I CHOSE RWY 12 AND LANDED. BEFORE TAXIING FOR DEP; ATIS STILL GAVE RWY 9L AS THE RWY IN USE. I ASKED GND FOR TAXIING INSTRUCTIONS AS I WAS UNFAMILIAR TO THE ARPT. THE WIND WAS STILL FAVORING RWY 12 AND ANOTHER ACFT HAD JUST DEPARTED RWY 12. I WAS UNDER THE IMPRESSION I WAS TO USE RWY 12. I DID THE RUNUP AND TAXIED UP TO THE HOLD LINE OF RWY 12 AND CALLED THE TWR AND TOLD THEM I WAS HOLDING SHORT OF THE ACTIVE TO TAKE OFF TO THE W. I WAS CLRED TO TAKE OFF. I TOOK OFF FROM RWY 12 AND WAS ABOUT 200-300 FT OFF THE GND WHEN THE CTLR NOTIFIED ME THAT I SHOULD HAVE TAKEN OFF FROM RWY 9L AND TO KEEP CLR OF THE DEP END OF RWY 9L AND HE WILL CALL MY TURN TO THE W WHICH HE DID AND I CONTINUED WITHOUT ANY FURTHER COMPLICATION. ON REFLECTION; BECAUSE I HAD BEEN GIVEN RWY 12 TO LAND EVEN THOUGH THEY WERE USING RWY 9L; AND THAT THE WIND FAVORED RWY 12 ALSO OTHER ACFT WERE DEPARTING RWY 12 AND THE TXWY THEY HAD GIVEN ME LED TO THE END OF RWY 12; I ASSUMED I WAS TO USE RWY 12. I SHOULD HAVE TOLD THE CTLR I WAS HOLDING SHORT OF RWY 12 AND FROM NOW ON I WILL ALWAYS QUOTE THE RWY NUMBER.