SMA DEPARTS WRONG RWY HAS CLOSE PROX WITH ARR ACFT AT ELM.

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I HAD ARRIVED AT ELM APPROX 2 HRS BEFORE THE INCIDENT; LNDG ON THE THEN-ACTIVE RWY 24. AT DEP TIME; WX CONDITIONS WERE ESSENTIALLY THE SAME AS AT ARR (3000 SCATTERED). AFTER LISTENING TO ATIS; WHICH DID NOT; AS I RECALL; RPT AN EXPECTED RWY; BUT DID GIVE A LOT OF INFO ABOUT CLOSED RWY 10/28 AND CLOSED TAXIWAYS DUE TO ARPT CONSTRUCTION; I CONTACTED GND CTL AND WAS INSTRUCTED TO TAXI TO 'RWY 6/24' INTXN WITH TAXIWAY CHARLIE; WHICH IS AT ABOUT THE MIDPOINT OF THE RWY. I WAS ASKED IF I WOULD ACCEPT AN INTXN TKOF; WHICH I ACCEPTED. AFTER RUNUP I CALLED TWR; RPTING READY TO TAKE OFF. I WAS CLRED FOR INTXN TKOF; WITH TFC RPTED AS AN SMA ON 3 MI FINAL. I RPTED 'ROLLING'; TOOK RWY 24; AND DEPARTED. ON CLBOUT I HEARD A GARBLED CONVERSATION BTWN TWR AND ANOTHER ACFT; THEN SPOTTED A HIGH-WING ACFT ABOUT 2 MI AHEAD; CIRCLING. I VOLUNTEERED TO THE TWR THAT I HAD TFC DEAD AHEAD. TWR THEN INFORMED ME THAT I HAD TAKEN OFF ON THE WRONG RWY; THAT I HAD BEEN CLRED FOR RWY 6 TKOF; NOT RWY 24. THE OTHER ACFT RPTED VISUAL CONTACT WITH ME; AND PASSED UNDER ME AND TO MY S. NEAREST PASSAGE WAS APPROX 700 FT. CONTRIBUTING FACTORS: 1) PLT ASSUMED THAT SIMILAR WX CONDITIONS MEAN SIMILAR WIND DIRECTION. 2) PLT NOT PAYING CLOSE ENOUGH ATTN. I STILL DO NOT RECALL BEING TOLD EXACTLY WHICH RWY TO USE BY ANY CTLR PRIOR TO TKOF. IF I WAS SO INSTRUCTED; I LOST IT IN THE BACKGND OF INTXN DEP QUERIES AND TFC ON FINAL RPTS. IF I WASN'T TOLD EXACTLY WHICH RWY TO USE; I SHOULD HAVE REQUESTED A VERIFICATION FROM THE CTLR. 3) INTXN TKOF FROM RWY MID-POINT ENHANCED CHANCE THAT PLT WOULD TKOF WRONG WAY; ESPECIALLY GIVEN THE CHANGE IN WIND DIRECTION FROM THE RECENT ARR. GUT INSTINCT WAS TO TKOF ON THE SAME RWY ON WHICH I HAD RECENTLY ARRIVED. RECOMMENDATION: TAXIWAY CLOSURE INFO AT ARPTS WITH OPERATING CTL TWRS DOES NOT BELONG ON ATIS. IT IS TWR'S RESPONSIBILITY TO DIRECT TFC ONTO THE APPROPRIATE TAXIWAYS. ARRIVING AND DEPARTING PLTS ONLY NEED TO KNOW CEILING; ALTIMETER; WIND; AND EXPECTED RWY.