SMA PLT HAS NMAC WITH COMMUTER AFTER EACH IS TOLD TO MAINTAIN VISUAL SEPARATION.

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WHILE LEVEL AT 3000 FT WAS ADVISED OF TFC AT 10 O'CLOCK AND 3500 FT; A COMMUTER. I RPTED TFC IN SIGHT. COMMUTER WAS ADVISED OF MY POS AND RESPONDED 'I SEE HIM OUT THERE.' WE WERE BOTH INSTRUCTED TO MAINTAIN VISUAL SEPARATION. I WAS PRECISELY MAINTAINING 3000 FT AND TRYING TO ASCERTAIN HIS EXACT FLT PATH TO DETERMINE IF THERE WAS A CONFLICT; HOWEVER; ASSUMING HE WAS AT 3500 FT. A 500 FT ALT DIFFERENCE IS NOT VERY DISCERNABLE FROM A DISTANCE SO I STAYED ON COURSE. HE WAS APPROACHING ME FROM MY L SO IF I TURNED L I WOULD BE DIRECTLY AT HIM ON HIS FLT PATH; AND AT A CONSIDERABLY SLOWER SPD. I STILL THOUGHT HE WAS AT 3500 FT AND HAD ME IN SIGHT AND WOULD TURN R TO PASS BEHIND ME IF HE FELT THERE WAS A CONFLICT. WHEN I REALIZED HE WAS DSNDING AND CONVERGING I STARTED TO TURN TO THE R. SEEMINGLY AT THE SAME TIME; HE REACTED AS IF HE HAD JUST SEEN ME AND TURNED R TO PASS BEHIND. I DON'T KNOW WHY HE HAD DSNDED OUT OF 3500 FT AFTER RPTING ME IN SIGHT AND BEING TOLD OF MY ALT. I ALSO CAN'T BELIEVE HE HAD ME IN SIGHT BECAUSE OF HIS ABRUPT EVASIVE MANEUVER. I DID NOT CONSIDER THE INCIDENT NOTEWORTHY BECAUSE I DIDN'T FEEL WE WERE DANGEROUSLY CLOSE AND ATTRIBUTED IT TO CONFUSED COMS. HE THEN RPTED TO THE CTLR THAT THERE WAS SOME BONANZA OUT HERE AND THAT I HAD JUST TURNED INTO HIS FLT PATH AND THAT WE CLRED EACH OTHER BY 200 TO 300 FT. HE EVEN ASKED THE CTLR TO VERIFY WITH ME THAT I HAD SEEN HIM. WHEN I TOLD THE CTLR WHAT MY ACTIONS WERE; THE COMMUTER PLT STATED THAT WASN'T WHAT REALLY HAPPENED. I WAS SURPRISED BY HIS VERSION OF WHAT HAPPENED AND WAS NOT GOING TO ARGUE WITH HIM ON THE RADIO. LESSONS LEARNED ARE THAT YOU CANNOT ASSUME ANYTHING; REGARDLESS OF THE AIRSPACE; EXPERIENCE OF THE PLTS; OR EVEN AFTER VERBAL CONFIRMATIONS ARE RECEIVED. CLOSURE RATES OF CONVERGING ACFT CAN BE GREAT; EVEN THOUGH THERE IS VERY LITTLE VISUAL RELATIVE MOTION. MY ASSUMPTIONS OF A 500 FT ALT CLRNC WAS NOT REAL. NOR WAS MY ASSUMPTION THAT HE HAD ME IN SIGHT AND THAT HE WOULD ALTER HIS COURSE SLIGHTLY TO PASS BEHIND ME. ON RETROSPECT; IN THE SAME SIT; I WILL HAVE THE CTLR VERIFY THE OTHER ACFT'S ALT AND CONFIRM THAT HE IS GOING TO STAY LEVEL UNTIL WE PASS.