I have a Garmin Forerunner 305. I've never paid much attention to the altitude until my century. I got a 6800 ft altitude change, and saw others as low as 5800 ft altitude change on the same ride. I have the autocorrect on because it is recommended. When I turned on elevation correction, it lowered it to 6300 ft.

I'm also curious what other do about this. How do I know what's accurate?