A combined altimeter/barometer instrument (1) is provided with setting means (6) which can be actuated in two different modes, i.e., in a pressure level setting mode so as to set the instrument to a known sea-level pressure, or in an altitude setting mode to a known altitude. Either setting mode will bring both displays to the correct value. However, one or another of the displays is 'locked' at the set value depending upon the setting mode used. Preferably, where the pressure setting mode is employed, the altitude display (4) is locked after setting, so that the instrument acts solely as a barometer to display changes in sea-level pressure, while continuing to display the altitude prevailing at the time of setting. Where the altitude setting mode is used, the sea-level pressure display is locked. The instrument acts as an altimeter and assumes that the sea-level pressure remains constant at the value prevailing during setting. In another embodiment, an altimeter instrument is provided with display means (14) which is locked or unlocked as a function of the output of a sensor (16, 18) which indicates whether the instrument or the apparatus whose altitude the instrument is sensing is in motion or about to be placed in motion or is stationary for a more than a predetermined period of time.