THE RPTR IS OBSERVED BY APCH CTL RADAR TO BE AT 3700 FT WHILE VISUAL AND ALTIMETER INDICATIONS WERE AGREEING AT 6000 FT.

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WHILE ON AN INST FLT PLAN; DURING DSCNT TO ROA; WE WERE GIVEN CLRNC TO DSND FROM CRUISING ALT OF 13000 FT MSL TO 6000 FT MSL. DURING THE PERIOD OF LEVEL FLT AT 6000 FT MSL; ATC CALLED THAT HE GOT AN ALT WARNING SHOWING US TO BE AT 3700 FT MSL. SOME HIGH TERRAIN IS LOCATED NEAR THIS ARPT; AS HIGH AS 4000 FT. HE EXPRESSED CONCERN AT OUR APPARENT ALT. I ASSURED ATC THAT WE WERE LEVEL AT 6000 FT MSL. SHORTLY THEREAFTER HE ASKED FOR MY NAME AND HOME BASE. SINCE WE WERE ON FINAL FOR RWY 33 AND DOING CHKLISTS I ASKED FOR A NUMBER TO CALL HIM AFTER LNDG. DURING THE PHONE CONVERSATION; HE SAID THAT A RECORD IS KEPT (IN ROA AREA) OF SUCH EVENTS. I GAVE HIM THE REQUESTED INFO AND SAID WE WOULD USE ANOTHER XPONDER ON DEP. I ASKED AND HE CONFIRMED THAT THERE WAS NO INDICATED CONFLICT WITH OTHER ACFT; ONLY THAT HIGH TERRAIN POINTS ARE CLOSE TO THE ARPT. DURING THE PHONE CONVERSATION; THE CTLR ASKED IF THIS HAPPENED BEFORE. I SAID IT HAPPENED ONCE. (IT WAS MY THOUGHT THAT THE FIRST ERROR IN ALT THE PREVIOUS WK WAS DUE TO EXCESSIVE RADAR INTERROGATIONS WHILE FLYING NEAR ACTIVE MIL EXERCISES. OUR XPONDER REPLY LIGHT WAS ON ALMOST CONTINUOUSLY.)