I have a Garmin Forerunner 301, but have a coworker who loves his GPSMap 60CS. I would think that with three or more satellites (creating a 3D perspective of your location) locked onto you, that calculation should be very accurate. However, my friend recently activated his 60CS during a cross-country airline flight and got weird numbers. His elevation was always shown as 3-4000 feet when he was actually at 30,000 feet.