ALTITUDE: HOW IS IT MEASURED?

Why is altitude inaccurate?

One reason altitude isn't accurate is because of geoids (read below), but there is another reason.

To be as accurate as the lat/lon data, you would need to receive satellite data from below you, which isn't possible when you're on the ground. In general, expect the altitude inaccuracy to be about twice that of the horizontal inaccuracy.

Altitude and geoids

A GPS device computes your altitude based on an ellipsoid model of the Earth which cannot accurately model the ups and downs of the Earth's surface. Most people expect altitude to be reported relative to Mean Sea Level (MSL), not the GPS ellipsoid.

To correct this, the GPS altitude calculation is adjusted to MSL by a table of "geoid" values covering the whole Earth. This table makes an approximate correction that permits a GPS device to report altitude relative to MSL.