Another thing about barometric altimeters is that they can become inaccurate due to calibration drift when the weather conditions change. Pilots know to regularly reset their altimeter calibration... oftentimes by listening to ATIS, ATC or an FSS weather report for the area within an immediate 100nm of their position. On the ground, when climbing and descending near mountains, you may encounter some steep pressure gradients simply due to the terrain's influence on the atmospheric conditions which can throw off your altitude readings.