I thought the altimeter would be more accurate than survey data. Survey data typically records a point every 30ish feet and interpolates the points in between. On the page for your ride, down in the Device Info section, there is a '?' next to Elevation Corrections. The note says that devices that contain an altimeter generally contain accurate elevation data so they disable elevation corrections by default.

One thing that could effect elevation data is drafting. Since an altimeter uses atmospheric pressure to determine altitude, drafting will affect it slightly because there is less air pressure in the draft than there is off the front. The difference is usually like 50-100 feet, though, in my experience.

neneboricua