Altimeters correct themselves by analyzing pressure data to identify underlying climatic changes and modify sensed pressures accordingly to provide corrected altitude values. The detected underlying climatic changes are used to provide a barometer function independent of altitude changes. Air pressure measured at a plurality of times is stored to provide historical pressure data. A reference pressure is modified in dependence on historical data to produce a modified reference pressure. An altitude value is obtained from the current air pressure value and the modified reference pressure and stored.