PLT CONFUSED AT RWY INTXN TAKES OFF ON TAXIWAY.

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PARKED AT RAMP AT EXTREME SE CORNER OF LA CROSSE MUNI ARPT. CONTACTED GND CTL AND CLRED TO TAXI AND HOLD SHORT OF RWY 3-36. COMPLIED. CALLED TWR AND RECEIVED CLRNC FOR TKOF WITH L TURN OUT. CROSSED RWY 'HOLD CLR' LINES; LINED UP; CALLED 'ROLLING'; ADDED PWR AND PROCEEDED TO TKOF. AT APPROX 400 FT AGL TWR ADVISED 'HOLD PRESENT ALT AND HDG.' I ACKNOWLEDGED. TWR THEN ASKED IF I KNEW THAT I HAD DEPARTED ON TAXIWAY AND IF I HEARD THE TKOF CLRNC CANCELLATION. I ANSWERED 'NO' TO BOTH QUESTIONS. ON HEARING THESE XMISSIONS; AN SMA WHICH HAD DEPARTED MOMENTS PRIOR; ADVISED TWR THAT THE RWY/TAXIWAY MARKINGS WERE VERY 'AMBIGUOUS.' THE REFERENCED 'HOLD SHORT' MARKINGS ARE SHORT OF THE TAXIWAY THAT PARALLELS RWY 3/21. THIS CONTRIBUTED TO MY ERROR. I NOTICED NOTHING DANGEROUS AS I HAD A CLR FIELD OF VIEW AND WAS AIRBORNE IN ABOUT 400 FT ON A LONG AND WIDE TAXIWAY. THIS WAS; HOWEVER; A SERIOUS ERROR ON MY PART. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. RPTR STATES HE WAS UNABLE TO GET ARPT DIAGRAM FROM FBO. THESE ARE USUALLY AVAILABLE IN THE PLTS BRIEFING ROOMS. FEELS IF HE HAD ONE HE WOULD NOT HAVE MADE THE MISTAKE. HE WAS CLRED FOR TKOF FROM THE RUNUP AREA. WOULD HAVE HAD TO CROSS RWY 36 TO GET TO RWY 3. WHEN HE SAW THE TAXIWAY THAT PARALLELS 3 HE TURNED AND TOOK OFF. FEELS A SIGN TO INDICATE AREA IS A TAXIWAY WOULD HELP AVOID CONFUSION.