GPS Altitude and Elevation: Why Height Is Less Accurate

GPS height readings are usually less accurate than the horizontal position, and the numbers can differ from the elevation shown on a map. There are two reasons: the geometry of satellites gives poorer vertical resolution, and GPS measures height relative to a mathematical model of the Earth that is not the same as sea level.

Why vertical error is larger

All satellites are above you. The receiver sees them spread across the sky, but none from below the horizon. That asymmetry means measurements constrain horizontal position better than vertical position. As a result, the vertical error of a GNSS fix is commonly larger than the horizontal error, often by a factor of about two. The same weakening of signals discussed in how GPS works applies.

Ellipsoid height vs elevation above sea level

The Earth is not a perfect sphere, and its surface at sea level is also not perfectly smooth. Two reference surfaces matter:

• The ellipsoid: a smooth, mathematically defined squashed sphere. GPS height is computed against this. For GPS, the ellipsoid is part of WGS 84.
• The geoid: a bumpy surface that approximates mean sea level, shaped by gravity. Elevations on topographic maps and in everyday life are usually measured from this surface.

The difference between the two, called the geoid separation, varies around the world and can reach tens of metres. A receiver that applies a geoid model will report height above sea level. One that does not will report ellipsoid height. If your reading looks off by a large, steady amount, this is a likely cause.

Barometers on phones

Many smartphones have a barometer. Air pressure falls as you go up, so the sensor can detect changes in height quite well, such as climbing a floor. It is usually combined with GPS to produce a smoother altitude. Pressure also changes with weather, so the absolute value may drift.

Practical advice

• Treat GPS altitude as an estimate, usually good to a few metres in the open and worse in cities or indoors.
• Compare differences in height over a short time, rather than trusting a single reading.
• Do not rely on GPS height to identify a floor or a flat; use the address detail instead.
• If you are recording elevations for work, check which height reference the device or software uses.

Where this meets addresses

Location codes such as UPCS codes describe a position on the ground in two dimensions, using latitude and longitude. They do not carry height, so floor or flat details still belong in the written address.