C172 HAD NMAC WITH C414 ON APCH FOR LNDG. SEE AND AVOID CONCEPT.

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AT THE TIME OF THE NEAR MISS; I WAS PERFORMING AN AERIAL TFC SURVEY. WE ARE TO REMAIN IN CONTACT WITH THE OHIO STATE TWR AND TO FLY OUTSIDE OF 270 AT 2000 FT MSL. THE RTE INVOLVES FLYING W ON HWY 270 THEN; FLYING SBOUND ALONG THE SAME ROAD. AT APPROX XXXX AN INFLUX OF ACFT BEGAN ARRIVING FROM THE N OF THE ARPT. 2 ACFT; A PIPER NAVAJO AND A CESSNA 414 BOTH RPTED INBOUND TO OSU AT APPROX THE SAME POS WITHIN (ABOUT 8 NM NW) 30 SECONDS OF EACH OTHER. BOTH ACFT WERE INSTRUCTED TO RPT A R BASE FOR RWY 27R AT HWY 315 AND HWY 270 WHICH IS STANDARD PROC. AT THIS TIME I WAS FOLLOWING HWY 71 N TO HWY 270 WHICH IS PART OF THE TFC SURVEY RTE; AND WAS DIRECTLY E OF THE RWY. A CONVERSATION QUICKLY ENSUED BTWN THE NAVAJO AND THE 414 AND THE TWR AS TO WHERE THE 2 ACFT WERE IN RELATION TO EACH OTHER. AFTER DETERMINING THAT NEITHER ACFT WAS A FACTOR TO EACH OTHER THE NAVAJO RPTED THE BASE AND WAS CLRED TO LAND. THE TWR CONTINUED TO POINT TFC OUT TO THE 414; HOWEVER NOTHING WAS EVER SAID TO ME OR TO HIM ABOUT EACH OTHER. AS I COME OVER TOP OF HWY 315 AND HWY 270 (WHERE THE X IS MARKED); AN ARRIVING ACFT WAS TOLD THAT THE 414 WAS AT HIGH STREET AND HWY 270; MY CURRENT POS. I LOOKED UP TO SEE THE 414 DSNDING IN FRONT OF ME; APPROX 5 FT IN FRONT OF ME AND 5 FT BELOW. I HAD NO TIME TO REACT; AND I DO NOT BELIEVE THE OTHER ACFT EVEN SAW ME. OHIO STATE TWR HAS A LINE FROM COLUMBUS APCH'S RADAR AND CAN IDENT TARGETS ON A RADAR SCOPE IN THE TWR.