AN A320 CREW; DEPARTING SEA; MISINTERPED ATC'S CLRNC AND TURNED TO THE WRONG HDG. A TCASII ALERT WITH AN EVASIVE MANEUVER WAS THE RESULT.

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WE WERE GIVEN THE SEA 3 DEP WHICH IS RADAR VECTORS TO J70. ON INITIAL CALL; WE WERE GIVEN TURN L OVER BOEING FIELD TO ELLIOT BAY. WE TURNED TO WHAT WE THOUGHT WAS ELLIOT BAY WHILE CLBING TO 9000 FT. ATC CALLED AND SAID WE HAD TURNED TO THE WRONG COURSE AND TO TURN TO 310 DEGS; LEVEL OFF AT 5000 FT. WHEN HE GAVE THIS CLRNC; WE WERE CLBING THROUGH 6400 FT. WE STARTED THE TURN TO 310 DEGS AND STARTED A DSCNT TO 5000 FT. IMMEDIATELY; WE RECEIVED A TCASII RA. IT TOLD US TO CLB. WE HAD THE TFC IN SIGHT. WE STOPPED OUR DSCNT AND CONTINUED OUR TURN TO MISS TFC. WE GOT WITHIN 1/2 MI OF THE OTHER ACFT. THE ACFT WAS AT 6000 FT AND ATC HAD VECTORED US INTO HIM. THIS WAS THE FIRST DAY OF A CAPT IOE. I WAS THE LCA FLYING. WHAT I HAD THOUGHT WAS ELLIOT BAY WAS ACTUALLY PUEGET SOUND. ELLIOT BAY WAS NOT ON THE SID OR AREA CHART. WE FOUND OUR MISTAKE ON ANOTHER PAGE FOR THE BAY VISUAL TO RWY 16. WE HAD TURNED 90 DEGS L WHEN THE CTLR WANTED ABOUT A 20 DEG L TURN. IF I HAD BEEN MORE FAMILIAR WITH SEA; OR IF ATC WOULD HAVE NOT USED VISUAL LANDMARKS FOR THE SEA 3 SID; THIS PROB WOULD NOT HAVE HAPPENED. AS HE HANDED US OFF; HE GAVE US HIGHER ALT AND TO INTERCEPT THE 230 DEG RADIAL. DIDN'T TELL OF WHICH VOR. WE ASSUMED THE 'SEA' RADIAL WHICH WOULD HAVE BEEN J70. ONLY PROB WAS; WE WERE ALREADY PAST THE RADIAL. WE WOULD HAVE HAD TO DO A U-TURN.