So on instruments-only how do you know how high off the ground you are?
Real aircraft? If so barometric altitude set to QNH (height above msl), flight level cruising set to standard pressure or more rarely, QFE (above aerodrome). That and minimum safe altitudes off the charts and plates. Other than the RA on bigger aircraft you dont really have an indication of exact altitude above the ground directly underneath you. You also dont need it.