60cs Altitude Calibration in GPS Posted March 30, 2005 I just recalibrated my barometer using the pressure given by www.weather.com. When I turned off the " Use with GPS" and inputted the pressure I "adjusted" for my actual altitude, I got a reading that was way too low. When I inputted the "given" pressure at sea level, my altitude was correct. I must conclude from that test that the GPSr takes the "actual" pressure at "actual" altitude, and subtracts it from the sea level pressure. It then determines the actual altitude from that result. Bottom line is get the sea level pressure from any of the above sources, and input it as your pressure at your location. The GPSr will do the rest automatically.