I know primary radar of ASR can determine only slant range, so by using Mode C transponder, ATC can know the altitude of aircraft.
And I found that if altimeter setting of radar is changed, displayed altitude also be changed.
So I guess that Mode C transponder can deliver only pressure information not altitude information. So Radar convert this pressure information to altitide information referring to radar altimiter setting. And it is not important what altimeter seeting pilot set.
Is it correct?