Two altitude readings, side by side
Most altitude tools show one number and leave you to guess where it came from. This page shows two. The brown sign is the ground elevation at your position, from a survey-based elevation model. The GPS altitude row below the signs is the height your device calculated from satellites, with its own accuracy estimate.
On the ground, outdoors, the two should be close, usually within 10 to 20 metres. If they differ a lot, the ground elevation is almost always the better number. If you are in an aircraft, on a tall building or on a drone controller, the GPS altitude is the one that reflects your own height, and the ground elevation tells you what is underneath.
When GPS altitude does not appear
Not every device reports altitude to the browser. Phones with GPS usually do once they have a satellite fix. Laptops and desktops generally do not, because they locate themselves from Wi-Fi, which carries no height information. In that case the row says your device did not report one, and the ground elevation on the sign is your answer.
On a phone, altitude often only appears when you are outdoors and the fix is good. Indoors, the position may come from Wi-Fi even on a phone, and the altitude will be missing or unreliable.
Altitude and elevation are not quite the same thing
In everyday speech people use them interchangeably. Technically, elevation is the height of a point on the ground above sea level, and altitude is the height of something above a reference, usually sea level but sometimes the ground. A plane flying at 35,000 feet has that altitude whatever the ground below it is doing; the airport it left from has an elevation.
For someone standing on the ground, the two are the same number. The elevation versus altitude guide covers the aviation and surveying terms if you need them.
Why altitude matters to your body, your cooking and your car
Air pressure falls as you go up: at 5,000 feet it is about 83 percent of the sea-level value, and at 10,000 feet about 69 percent. Less pressure means less oxygen in each breath, which is why exercise feels harder in Denver than in Miami, and why some people get headaches or trouble sleeping on their first nights above 8,000 feet.
Lower pressure also lowers the boiling point of water, by roughly 1°F for every 500 feet. That changes cooking times, canning safety rules and baking chemistry. Engines without turbochargers lose around 3 percent of their power per 1,000 feet, and a golf ball or baseball carries noticeably further.
If you are planning a trip to high altitude and have a heart or lung condition, talk to a doctor before you go. This page gives you the number; it is not medical advice.