SMA TOOK OFF ON WRONG; POSSIBLY CLOSED RWY AT ISP.

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I TAXIED ON TAXIWAY S AND E FOLLOWING GND INSTRUCTIONS TO TAXI TO 33L. LIGHTED 33L RWY SIGN WAS NOTED AT THE RUNUP AREA AND ON BEING CLRED FOR TKOF I TOOK OFF ON WHAT I THOUGHT WAS 33L. AFTER BECOMING AIRBORNE I WAS TOLD BY THE TWR THAT I HAD TAKEN OFF ON THE WRONG RWY AND TO MAKE A R TURN. ON RETURNING HOME I TELEPHONED AS INSTRUCTED AND SPOKE WITH PERSON WHO SAID THAT HE WAS THE CTLR ON DUTY WHEN I TOOK OFF AT ISLIP ON WHAT HE SAID WAS THE WRONG RWY. HE SAID THAT OTHER PEOPLE HAVE DONE THE SAME THING AND I TOLD HIM THAT IT WAS NOT VERY WELL MARKED. CALL TO ISLIP (KISP) TWR SUPVR. I DISCUSSED THE TKOF PROBLEM WITH HER AND TOLD HER THAT IT WAS CONFUSING AT THE INTXN OF 33L; 28 AND TAXIWAY E. SHE MENTIONED THAT THEY HAD EXPERIENCED SIMILAR MISUNDERSTANDINGS IN THE PAST WITH PEOPLE TAKING OFF ON THE WRONG RWY. WHEN I ASKED HER IF THERE WAS ANYTHING THAT I COULD DO TO HELP THE SITUATION SUCH AS WRITING A LETTER TO THE ARPT MGR SHE SAID THAT WAS NOT NECESSARY BECAUSE THEY WERE AWARE OF THE PROBLEM AND SOME CORRECTIVE STEPS WERE PRESENTLY BEING TAKEN. RECEIVED TELEPHONE CALL FROM FSDO AND HE SAID THAT THEY WERE FILING A RPT ON THE LNDG INCIDENT BUT THAT THE TWR WAS NOT FILING SOMETHING ON THE TKOF. I EXPLAINED TO HIM THAT THE TWR TOLD ME THAT THEY WERE AWARE THAT A CONDITION EXISTED AT THE 33L; 28 TAXIWAY E INTXN THAT HAD CAUSED CONFUSION IN THE PAST CORRECTIVE STEPS WERE IN PROCESS. THE EVENTS IN MY OPINION WERE THE RESULT OF THE TWR CTLRS' FAILURE TO RECOGNIZE UNUSUAL SITUATIONS THAT COULD PRESENT POTENTIAL PROBLEMS; CTLRS' FAILURE TO ADEQUATELY COMMUNICATE ALSO THE LACK OF SIGNS TO PROVIDE RWY IDENT AND TO WARN OF UNUSUAL FIELD CONDITIONS. SUGGESTIONS TO PREVENT A RECURRENCE: AT RWY INTXN TKOFS GIVE 'POS AND HOLD' INSTRUCTIONS BEFORE CLRING FOR TKOF TO MAKE SURE THE ACFT IS POSITIONED ON THE CORRECT RWY. PROVIDE BETTER MARKINGS AND SIGNS TO PROPERLY IDENT RWYS THAT ARE OTS.