Reason one: GPS is weaker at height
Satellite positioning works out where you are from the timing of signals from satellites spread across the sky. For horizontal position, satellites in every direction help pin you down. For height, every satellite is above you and none is below, so the geometry is lopsided and the vertical answer is less certain.
As a rule of thumb, vertical error is about one and a half to two times the horizontal error. A phone that shows its position to within 5 metres outdoors may be 10 metres out on height, and indoors or among tall buildings far more. That is why an altitude reading can wander up and down while you stand still.
Reason two: height above what?
Sea level is not a simple surface. Gravity varies around the Earth, so the average sea surface, extended under the land, is lumpy. Surveyors call that surface the geoid, and elevations above sea level are measured from it.
GPS does not use the geoid. It works with a smooth mathematical shape called an ellipsoid, the WGS 84 ellipsoid, and the raw height it computes is height above that. Across most of the United States the geoid sits roughly 10 to 50 metres below the ellipsoid, commonly around 30 metres. So a raw GPS height reads about that much lower than the true elevation.
The web standard browsers follow defines altitude as height above the WGS 84 ellipsoid. In practice devices differ: iPhones generally report height above sea level, while many Android devices report height above the ellipsoid. That alone can make an Android phone read around 100 feet too low in much of the US.
This is why our tool shows ground elevation from a survey model as the main number, and your device's GPS altitude separately below it.
What about the barometer in my phone?
Many phones, including every iPhone since the iPhone 6, have a barometric pressure sensor. Air pressure falls steadily as you climb, so the barometer is excellent at measuring changes in height, such as the number of floors you have walked up.
It is not good at absolute height on its own, because air pressure also changes with the weather. A passing front can move the pressure by as much as a climb of 100 metres. Fitness trackers and hiking apps combine the two: GPS or a map for the starting height, the barometer for changes along the way.
How to get a trustworthy elevation
- Look up the ground elevation from a digital elevation model, which is what our elevation tool does. In the US the model is based on survey data and is usually good to a metre or two.
- For a specific building or legal purpose, use a surveyed benchmark or a surveyor's elevation certificate.
- If you must use GPS altitude, take the reading outdoors, wait until the accuracy settles, and treat anything within 20 metres of the model value as a match.
- On a hiking GPS unit or watch, calibrate the altimeter at a known elevation, such as a trailhead sign, before setting out.