Altimeters are often misunderstood because they do not measure your actual position on the earth (true altitude) such as a GPS, but measure the air pressure and predict the altitude based on this measurement. This is referred to as "Pressure Altitude". Pressure Altitude is profoundly affected by the true altitude and to a lesser extent, changes in the weather, often referred to as barometric pressure changes. Thus, your Pressure Altitude can change without changing your position. These changes can be as great as a few hundred feet (the changes due to weather are more pronounced at higher altitude). To offset the difference between Pressure Altitude and True Altitude, you must calibrate the device at a known altitude, whenever convenient.