Barometric AltimeterCheck out the definitions at Wikipedia for an altimeter and barometer.GPS provides Altitude information when it is able to communicate with 4 or more satellites. This is referred to as Three Dimensional (3D) signal. Latitude, Longitude and Altitude are the three dimensions. If you want to get technical then the fourth dimension would be time.The big question most people ask is "Why do I need a barometric altimeter when I have GPS altitude?" A barometric altimeter uses the atmospheric pressure to determine your altitude above sea level. It turns out they are able to determine altitude within +/- 3 vertical meters or so. The altimeter will detect your change in altitude based on the change in the atmospheric pressure (the higher you go the less pressure).The problem with a barometric altimeter is the requirement to calibrate it every time you start your activity. If you don't the altimeter will still work and relative measurements will be accurate, but absolute measurements will be off. This means that your cumulative elevation gain/loss will probably be correct, but all of the elevations along the way will be offset by the difference of the actual altitude and the altitude on the device.GPS altitude doesn't need any calibration, but for complex reasons the GPS unit is not able to determine elevation as accurately as a barometric altimeter.Combining both GPS and barometric altimeters, Garmin GPS units are able to provide the most accurate altitude readings of any handheld device. Absolute location is provided originally by the satellite to help auto-calibrate the barometric altimeter, then the barometric altimeter is used to provide a more stable elevation change. The GPS device will constantly calibrate the barometric altimeter throughout an activity because the pressure may change due to weather conditions. This is a great advantage during long days of hiking, biking or running.
Lucky7777 How does the altimeter work?I set precise level of level. Altimeter->Calibrate->Set level.In 24 hours I came to the same spot and difference was 57 ft.How does the altimeter work, how precise is it? What factors can influence the measurement? Posted Jan 15, 2009 8:41 pmGPSFix re: How does the altimeter work?The barometric altimeter is very precise but keep in mind there are two factors that can influence the readings:- Elevation (obviously!)- Changes in atmospheric pressure (caused by weather changes)I find that over relatively short time periods (several hours) that if I manually calibrate the altimeter AND turn off auto-calibration that the barometer is very accurate (within 5-10')That brings up auto-calibration. Auto-calibration uses GPS elevation to calibrate the barometric altimeter. GPS elevation is visible on the satellite page. The issue with GPS elevation is that isn't very accurate even under the best satellite conditions with WAAS turned on. You still will probably have 20-30' error and in the woods 50-70' is more typical.What does this all mean? If you have auto-calibration turned on (which is the case by default) you'll see errors in the 50-100' range depending on GPS signal quality and how fast the weather is changing over time. I'm guessing this is what you observed. The up side is that you don't need to remember to manually calibrate and your altimeter will always be somewhat accurate.If you really want accurate altimeter readings though you'll need to disable auto-calibration and use manual calibration to some known good reference (topo map, survey mark, etc). But after several hours, again depending on weather, your barometer can drift requiring manual calibration.