GPS receiver as altimeter

The elevation or altitude calculated by a GPS receiver from satellite data isn’t very accurate. Because of this, some GPS units have altimeters, which provide the elevation, ascent/descent rates, change in elevation over distance or time, and the change of barometric pressure over time. (The rough-and-ready rule is that if barometric pressure is falling, bad weather is on the way; if it’s rising, clear weather is coming.) Calibrated and used correctly, barometric altimeters can be accurate within 10 feet of the actual elevation. Knowing your altitude is useful if you have something to reference it to, such as a topographic map. Altimeters are useful for hiking or in the mountains. On GPS units with an electronic altimeter/barometer, calibrating the altimeter to ensure accuracy is important. To do so, visit a physical location with a known elevation and enter the elevation according to the directions in your user’s guide.

Airports are good places to calibrate your altimeter or get an initial base reading; their elevation is posted for pilots to calibrate their airplanes’ altimeters. If you’re relying on the altimeter/barometer for recreational use, I recommend calibrating it before you head out on a trip. Some GPS receivers have features that allow you to increase the accuracy of your location by using radio signals not associated with the GPS satellites. If you see that a GPS receiver supports WAAS or Differential GPS, it has the potential to provide you with more accurate location data.