Altimeter: What Is My Elevation?

How a GPS altimeter measures elevation

To fix your position, your phone measures its distance to several satellites at once. Solving those distances in three dimensions gives your altitude as well as your latitude and longitude. The altitude is reported above a model of mean sea level, so it reads like the heights printed on maps and mountain signs.

Vertical accuracy from GPS is usually about one and a half times worse than horizontal accuracy, because all the satellites are above you rather than around you. Expect around 5 to 15 metres outdoors on a modern phone. The accuracy readout shows the margin your device reports for each reading.

GPS altitude versus a barometric altimeter

Hiking watches and aircraft often use air pressure to estimate altitude. Pressure sensors respond quickly to small changes but drift with the weather and need calibrating against a known height. GPS altitude does not drift with the weather, but each reading is noisier. Watching the lowest and highest values over a few minutes gives a good sense of the true figure.

Why some devices show no altitude

Laptops and desktops usually estimate position from Wi-Fi networks and cannot calculate height. Some browsers also omit altitude even on phones. If the reading stays blank, open the page on a phone outdoors. For your coordinates rather than height, use My Location.

Where elevation matters

Air gets thinner as you climb. At 2,500 metres there is roughly a quarter less oxygen in each breath than at sea level, which is why hikers and skiers can feel short of breath or get headaches when they arrive somewhere high. Knowing your elevation helps you pace a climb and decide when to rest.

Elevation also changes everyday things. Water boils at a lower temperature as you go higher, about 1 °C lower for every 300 metres, so pasta takes longer to cook in the mountains. Baking recipes, tyre pressures and even some medical devices come with high-altitude adjustments. Pilots, drone operators and surveyors work with altitude constantly, though they rely on certified instruments rather than a phone.