The problem with elevations from GPS chips are that they are ellipsoidal heights, that is the height from the surface of the ellipsoid used to model the earth.

The problem with elevations from GPS chips are that they are ellipsoidal heights, that is the height from the surface of the ellipsoid used to model the earth. This can be very far from the elevation referred from mean sea level. I think SRTM is still the best way to get elevation data as mentioned by other commenters.