Geometric vs. Barometric Altitude

There are two main ways of determining altitude: geometric measurement and barometric measurement.

Geometric Measurement

Geometric altitude measurement is typically performed using GPS, although there are other methods which would give the same result. A geometric altitude measurement tells you exactly how far you are from the ground. This is the measurement you would expect to get if you had a very long ruler.

Barometric Measurement

In barometric altitude measurement, we start with a measurement of air pressure.

http://www.xcmag.com/2011/07/gps-versus-barometric-altitude-the-definitive-answer/