PA32 PLT HAS A RWY INCURSION AT TPA.

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I WAS LOCATED AT AN FBO AT TPA. I COPIED CURRENT ATIS AND SWITCHED TO CLRNC AND OBTAINED MY IFR CLRNC. UPON COMPLETION OF COPYING MY CLRNC I CONTACTED GND CTL ON 121.7 AND RECEIVED A CLRNC TO TAXI TO RWY 18L FOR DEP. AS I STARTED OUT I BELIEVE GND ASKED ME IF AN INTXN DEP WOULD BE ACCEPTABLE. I THOUGHT IT WAS AN INTXN FROM THE ACTIVE DEP RWY OF RWY 18L SO I CONTINUED MY TAXI TOWARDS RWY 18L. I DID NOT RECALL GND CHANGING MY DEP TO AN INTXN DEP FOR RWY 9. SO I CONTINUED MY TAXI TO RWY 18L BELIEVING I WAS CLRED TO RWY 18L AND NOT TOLD TO HOLD SHORT OF RWY 9 I PROCEEDED TO CROSS RWY 9 ENRTE TO RWY 18L. IT WAS AT THIS TIME THAT GND HAD ME STOP MY PROGRESS IN THE MIDDLE OF RWY 9 AND TO IMMEDIATELY CONTACT TWR ON 119.5 FOR MY DEP CLRNC. IN THE CONFUSION I ADVISED GND THAT I BELIEVED I WAS STILL ENRTE TO RWY 18L; BUT WOULD HOLD PRESENT POS AND CONTACT TWR. TWR IMMEDIATELY CLRED ME FOR AN INTXN DEP OFF RWY 9. I BELIEVE THE PROB AROSE DUE TO MISCOM BTWN PLT AND GND CTL. I WAS ORIGINALLY CLRED TO TAXI RWY 18L WITH NO HOLD SHORT INSTRUCTIONS AND WHEN HE GAVE ME THE INTXN DEP I FAILED TO REALIZE IT WAS AN INTXN DEP OFF RWY 9; NOT RWY 18L WHICH I HAD WRITTEN DOWN ON MY NOTE PAD. I BELIEVE THAT I SHOULD HAVE DOUBLECHKED MY TAXI INSTRUCTIONS SINCE I WAS UNFAMILIAR WITH THE ARPT AND AS I WAS TAXIING TO RWY 18L I HAD QUESTIONED MYSELF; BUT DID NOT QUERY ATC ON THE DEP INTXN.