Why is the altitude on my GPS different than the altitude on my altimeter?
Regarding the altitude discrepancy, keep in mind that your altimeter does not measure altitude. What the altimeter measures is air pressure. Your altimeter is calibrated to display a certain change in altitude based on a change in air pressure. For every inch of mercury pressure change, the altimeter displays a change of approx. 1000 ft. of altitude. Think of it this way. If the plane is sitting on the ramp and shows the correct altitude when the outside air pressure is 30.00, it will show an altitude of ramp+250 if the outside air pressure drops to 29.75. It is still on the ramp, but the altimeter changed because the pressure changed. The standard lapse rate (SLR) for air pressure is 1-inch pressure = 1,000-ft altitude and this is how your altimeter is calibrated (approximately). However, when is anything in nature a perfect standard? What the SLR implies is that 1-inch will USUALLY equal 1,000-ft. depending on altitude. The higher you go, the less accurate this is. However, this is not a problem since everyone uses the same system. If everyone's altimeter is set to the same pressure setting, then they will all be off the same amount and vertical separation will be ensured. The GPS does not use air pressure to determine altitude. As such, it is not as apt to have pressure errors in determining altitude. What this means is that your GPS derived altitude is almost always more accurate than your altimeter. However, since everyone uses a pressure-based system to determine altitude, you need to fly using the pressure altimeter to ensure the required vertical separation.
