B737 LANDS ON PROTECTED; DISPLACED THRESHOLD 2000 FT FROM RWY APCH END INSTEAD OF AT THE 3000 FT MARKER WHERE THE INTENDED RWY BEGINS.

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LNDG IN MEDFORD; OR; IN LOW LIGHT CONDITIONS (DUSK). THE THRESHOLD ON RWY 32 WAS TEMPORARILY DISPLACED 3000 FT FOR CONSTRUCTION. THERE WERE BARRICADES IN PLACE 2000 FT FROM THE ORIGINAL THRESHOLD; PROVIDING A 1000 FT SAFETY ZONE FOR THE WORKERS IN THE CONSTRUCTION AREA. THE CAPT THOUGHT THE BARRICADES MARKED THE NEW THRESHOLD. WE SKIMMED OVER THE BARRICADES AT WHAT I CONSIDERED 'A LITTLE TOO LOW;' BUT I THOUGHT THE CAPT WAS ATTEMPTING TO LAND 'ON THE NUMBERS' OF THE NEW THRESHOLD. THIS SEEMED PRUDENT TO ME SINCE THE RWY WAS NOW SHORTENED TO 5200 FT -- VERY SHORT FOR A B737! WE TOUCHED DOWN ABOUT AN AIRPLANE'S LENGTH SHORT OF THE REAL DISPLACED THRESHOLD. THERE WAS NO RISK OF DAMAGE OR INJURY; BUT THIS WAS A CLR CASE OF MISCOM. I KNEW WHERE THE REAL DISPLACED THRESHOLD WAS AND JUST ASSUMED THE CAPT DID ALSO. I COULD HAVE CONFIRMED THAT HE KNEW ABOUT THE BARRICADES AT 2000 FT AND THE NEW THRESHOLD AT 3000 FT. ALSO; THE NEWLY PAINTED THRESHOLD WAS DIFFICULT TO SEE; ESPECIALLY COMING IN ON A FLAT APCH (THERE WAS NO PAPI OR VASI).