SH36 FLT CREW IS QUESTIONED BY IND TRACON REGARDING THEIR ALT INDICATION WHICH WAS 300 FT ABOVE THE ASSIGNED 5000 FT; TCASII TA RECEIVED.

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ON CLBOUT FROM RWY 23R MY CAPT AND I WERE GIVEN A HDG OF 320 DEGS UP TO 5000 FT MSL. WE WERE TOLD TO STOP AT 5000 FT IN ORDER TO LET XING TFC CLR WHICH WAS AT AN ALT OF 6000 FT MSL HDG WBOUND. THE ACFT'S POS WAS AT OUR 1 O'CLOCK POS; SHOWING +800 FT ABOVE US BASED ON OUR TFC ALERT TCASII SYS. UPON REACHING 5000 FT; THE INDIANAPOLIS DEP CTL GAVE US A WARNING SAYING THAT OUR ACFT WAS SHOWING 5300 FT MSL; AND WE WERE APCHING THE WBOUND TFC. AT THE PRESENT TIME OUR ALT WAS READING 5100 FT MSL WHICH WAS CALIBRATED WITHIN 75 FT AT SEA LEVEL TO GIVE US A CORRECT ALT READING OF 5000 FT MSL. AT THIS EXACT TIME OF THE WARNING THE WBOUND TFC WAS STILL AT 6000 FT MSL AND OUR 5 O'CLOCK POS. ALSO THE WBOUND TFC AT THAT SAME TIME CALLED INDIANAPOLIS DEP STATING THAT TFC WAS IN SIGHT AND NO FACTOR. THE ENDING RESULT WAS THAT NO EVASIVE ACTION OR ACFT DEV WAS NECESSARY BY EITHER ACFT. THE ENDING FACTOR COULD HAVE BEEN A FALSE ALT ALERT ON THE INDY DEP RADAR OR THE ALTIMETERS GIVING A FALSE READOUT WHICH NEEDS TO BE CHKED.