Altimeter: how high am I?
Tap “Measure altitude” and allow location access. The phone provides the GPS altitude with its accuracy; over several readings a weighted average is formed.
How it is calculated
How to measure your altitude
- Tap “Measure altitude” and allow location access.
- Wait a minute or two outdoors with a clear view of the sky. The accuracy shows how reliable the value is.
- The average combines the last 20 readings and weights more accurate ones more heavily. Minimum and maximum are handy on hikes.
GPS altitude is not sea level
According to the W3C Geolocation API, the browser reports the height above the WGS 84 ellipsoid, a mathematical ideal shape of the Earth. Height above sea level refers instead to the geoid, the surface of equal gravity. The two differ depending on location – worldwide by up to about 106 m (Indian Ocean). The rule is: sea-level height ≈ GPS height − geoid height. If you enter the geoid height for your location, the tool converts it; national survey agencies and NOAA’s geoid models provide such values.
Conversion to feet: 1 ft = 0.3048 m (NIST).
Limits
- GPS usually measures height less accurately than position; watch the accuracy shown. The average smooths noise but does not remove systematic error.
- Many computers and some browsers provide no altitude at all.
- Not suitable for aviation, mountain safety or surveying.
Frequently asked questions
How accurate is the altitude from my phone?
GPS altitude fluctuates more than position. The accuracy shown and the average over several readings help you judge it.
Why does GPS altitude differ from the map?
GPS refers to the WGS 84 ellipsoid, maps to sea level (the geoid). The difference is the geoid height, which varies by location.
Why does my laptop show no altitude?
Without a GPS receiver the browser cannot determine altitude. Use a phone or tablet and go outdoors.
What is the geoid height?
The distance between geoid and ellipsoid at your location. With it, GPS height converts to sea-level height: sea level ≈ GPS height − geoid height.
Is my location stored?
No. Position and altitude are only shown in your browser and are neither stored nor sent.
Sources and legal basis
- W3C – Geolocation API
- MDN Web Docs – GeolocationCoordinates.altitude
- NOAA NGS – Geoid models
- Wikipedia: Geoid
- NIST Special Publication 811 – Guide for the Use of the SI (conversion factors)
As of:
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