The Garmin 496 GPS-derived altitude is a helluva lot more accurate than an altimeter, especially with EGNOS.
Though I seem to remember something (from "satellite and Microwave systems & radio Navigation" from 20 years ago at University !!
) about GPS receivers showing geodetic altitude or ellipsoidal altitude which uses a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. The problem is that in places like the Himalayas where you have lots of mass, this model creeps up above the true surface of the earth due to the additional mass. In some other places is becomes lower than sea level due to the opposite.
So I wouldn't trust the GPS altitude under 100' agl that is for sure!