An altimeter does not measure altitude on the Garmin GPS device. The altimeter in the aircraft measures air pressure. The 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. The GPS does not use air pressure to determine altitude and is not subject to the pressure errors in determining altitude. Therefore, the GPS derived altitude is almost always more accurate than an altimeter. Since everyone uses a pressure-based system to determine altitude, it is integral to utilize a pressure sensitive altimeter to ensure required vertical separation.